As the other answers have stated, the term "ionizing radiation" has a specific technical meaning, and its use is restricted to radiation where a single photon at the radiation's frequency has enough energy $E=h\nu$ to ionize biological tissue.

However, it is important to note that radiation of all frequencies can produce ionization if its intensity is strong enough. In the literature this is known as multiphoton ionization, and you can think of it heuristically as two or more photons being present at the same time and being absorbed simultaneously.

This does have a price, of course, because the fact that you need those two or more photons to be 'present at the same time' (or, in more technical language, that you're working in perturbation theory of order $\geq 2$) means that the light source needs to be intense enough to allow for that simultaneous presence. This ends up meaning that for an $n$-photon process the ionization rate scales with the light intensity $I$ as $I^n$,* which is a punitively high scaling for any light source that's not a laser.

However, once we did develop lasers, in 1960, it took just five years for the first example of multiphoton ionization to be described, taking the record from single-photon to seven photons all in one go [JETP Lett. 1, 66 (1965)]. In the decades since, multiphoton ionization has been an important tool in the toolbox of atomic and molecular physics for our understanding of the structure and dynamics of matter.

On the other hand, the requirement that the intensity be high does mean that you need specialized experiments to show the effect, which means that ionization by low-frequency radiation is negligible in everyday life $-$ which is why the term 'ionizing radiation' is restricted. In essence, the fact that the ionization rate scales linearly with the intensity means that the total number of ionization events (together with the biological damage it causes if one assumes a linear biological response, which may or may not be a good model) will only depend on the total energy that passes through the tissue, and not the rate at which it is delivered. Thus, if you dilute the dose by turning the intensity down by half but keeping it on for twice as long (so that the total energy in the radiation is constant), then the single-photon ionization will stay constant, but $n$-photon processes will go down by $1/2^n$.

That said, if you want a reasonably accessible demonstration of ionization with a long-wavelength light source, the thing to search for is light-induced optical breakdown in air (example), in which a long-wavelength laser is used to ionize air at its focus. This is mostly an avalanche phenomenon (see this answer for more details) but the seed electrons are often produced via multiphoton ionization.

^{* for intensities that are low enough for perturbation theory to hold. Most of the interesting physics in multiphoton ionization occurs in non-perturbative regimes where things are more complex. The introduction of my PhD thesis has more details on what that looks like.}

As the other answers have stated, the term "ionizing radiation" has a specific technical meaning, and its use is restricted to radiation where a single photon at the radiation's frequency has enough energy $E=h\nu$ to ionize biological tissue.

However, it is important to note that radiation of all frequencies can produce ionization if its intensity is strong enough. In the literature this is known as multiphoton ionization, and you can think of it heuristically as two or more photons being present at the same time and being absorbed simultaneously.

This does have a price, of course, because the fact that you need those two or more photons to be 'present at the same time' (or, in more technical language, that you're working in perturbation theory of order $\geq 2$) means that the light source needs to be intense enough to allow for that simultaneous presence. This ends up meaning that for an $n$-photon process the ionization rate scales with the light intensity $I$ as $I^n$,* which is a punitively high scaling for any light source that's not a laser.

However, once we did develop lasers, in 1960, it took just five years for the first example of multiphoton ionization to be described, taking the record from single-photon to seven photons all in one go [JETP Lett. 1, 66 (1965)]. In the decades since, multiphoton ionization has been an important tool in the toolbox of atomic and molecular physics for our understanding of the structure and dynamics of matter.

On the other hand, the requirement that the intensity be high does mean that you need specialized experiments to show the effect, which means that ionization by low-frequency radiation is negligible in everyday life $-$ which is why the term 'ionizing radiation' is restricted. In essence, the fact that the ionization rate scales linearly with the intensity means that the total number of ionization events (together with the biological damage it causes if one assumes a linear biological response, which may or may not be a good model (example)) will only depend on the total energy that passes through the tissue, and not the rate at which it is delivered. Thus, if you dilute the dose by turning the intensity down by half but keeping it on for twice as long (so that the total energy in the radiation is constant), then the single-photon ionization will stay constant, but $n$-photon processes will go down by $1/2^n$.

That said, if you want a reasonably accessible demonstration of ionization with a long-wavelength light source, the thing to search for is light-induced optical breakdown in air (example), in which a long-wavelength laser is used to ionize air at its focus. This is mostly an avalanche phenomenon (see this answer for more details) but the seed electrons are often produced via multiphoton ionization.

^{* for intensities that are low enough for perturbation theory to hold. Most of the interesting physics in multiphoton ionization occurs in non-perturbative regimes where things are more complex. The introduction of my PhD thesis has more details on what that looks like.}

As the other answers have stated, the term "ionizing radiation" has a specific technical meaning, and its use is restricted to radiation where a single photon at the radiation's frequency has enough energy $E=h\nu$ to ionize biological tissue.

However, it is important to note that radiation of all frequencies can produce ionization if its intensity is strong enough. In the literature this is known as multiphoton ionization, and you can think of it heuristically as two or more photons being present at the same time and being absorbed simultaneously.

This does have a price, of course, because the fact that you need those two or more photons to be 'present at the same time' (or, in more technical language, that you're working in perturbation theory of order $\geq 2$) means that the light source needs to be intense enough to allow for that simultaneous presence. This ends up meaning that for an $n$-photon process the ionization rate scales with the light intensity $I$ as $I^n$,* which is a punitively high scaling for any light source that's not a laser.

However, once we did develop lasers, in 1960, it took just five years for the first example of multiphoton ionization to be described, taking the record from single-photon to seven photons all in one go [JETP Lett. 1, 66 (1965)]. In the decades since, multiphoton ionization has been an important tool in the toolbox of atomic and molecular physics for our understanding of the structure and dynamics of matter.

On the other hand, the requirement that the intensity be high does mean that you need specialized experiments to show the effect, which means that ionization by low-frequency radiation is negligible in everyday life $-$ which is why the term 'ionizing radiation' is restricted. In essence, the fact that the ionization rate scales linearly with the intensity means that the biological response is only dependent on the radiation dose, and not how it is delivered: if you dilute the dose by turning the intensity down by half but keeping it on for twice as long, so that the total energy in the radiation will be constant, then the single-photon ionization will stay constant, but $n$-photon processes will go down by $1/2^n$.

That said, if you want a reasonably accessible demonstration of ionization with a long-wavelength light source, the thing to search for is light-induced optical breakdown in air (example), in which a long-wavelength laser is used to ionize air at its focus. This is mostly an avalanche phenomenon (see this answer for more details) but the seed electrons are often produced via multiphoton ionization.

^{* for intensities that are low enough for perturbation theory to hold. Most of the interesting physics in multiphoton ionization occurs in non-perturbative regimes where things are more complex. The introduction of my PhD thesis has more details on what that looks like.}

As the other answers have stated, the term "ionizing radiation" has a specific technical meaning, and its use is restricted to radiation where a single photon at the radiation's frequency has enough energy $E=h\nu$ to ionize biological tissue.

However, it is important to note that radiation of all frequencies can produce ionization if its intensity is strong enough. In the literature this is known as multiphoton ionization, and you can think of it heuristically as two or more photons being present at the same time and being absorbed simultaneously.

This does have a price, of course, because the fact that you need those two or more photons to be 'present at the same time' (or, in more technical language, that you're working in perturbation theory of order $\geq 2$) means that the light source needs to be intense enough to allow for that simultaneous presence. This ends up meaning that for an $n$-photon process the ionization rate scales with the light intensity $I$ as $I^n$,* which is a punitively high scaling for any light source that's not a laser.

However, once we did develop lasers, in 1960, it took just five years for the first example of multiphoton ionization to be described, taking the record from single-photon to seven photons all in one go [JETP Lett. 1, 66 (1965)]. In the decades since, multiphoton ionization has been an important tool in the toolbox of atomic and molecular physics for our understanding of the structure and dynamics of matter.

On the other hand, the requirement that the intensity be high does mean that you need specialized experiments to show the effect, which means that ionization by low-frequency radiation is negligible in everyday life $-$ which is why the term 'ionizing radiation' is restricted. In essence, the fact that the ionization rate scales linearly with the intensity means that the total number of ionization events (together with the biological damage it causes if one assumes a linear biological response, which may or may not be a good model) will only depend on the total energy that passes through the tissue, and not the rate at which it is delivered. Thus, if you dilute the dose by turning the intensity down by half but keeping it on for twice as long (so that the total energy in the radiation is constant), then the single-photon ionization will stay constant, but $n$-photon processes will go down by $1/2^n$.

That said, if you want a reasonably accessible demonstration of ionization with a long-wavelength light source, the thing to search for is light-induced optical breakdown in air (example), in which a long-wavelength laser is used to ionize air at its focus. This is mostly an avalanche phenomenon (see this answer for more details) but the seed electrons are often produced via multiphoton ionization.

^{* for intensities that are low enough for perturbation theory to hold. Most of the interesting physics in multiphoton ionization occurs in non-perturbative regimes where things are more complex. The introduction of my PhD thesis has more details on what that looks like.}