Sparknotes $\qquad $
Quora answer by Mr. Kennan Hye BS (Engineering), MEng. (Acoustics):
Pitch is sometimes defined as the fundamental frequency of a sound wave (i.e. generally, the lowest frequency in a given sound wave). For most practical purposes, this is fine, and pitch and frequency can be thought of as equivalent. On the other hand, for most practical purposes, amplitude can be thought of as volume.
However, technically, pitch (and volume) are human perceptions. Thus, our perception of pitch and volume are not solely based on frequency and amplitude respectively, but are based on a combination of both (and even other factors). Frequency overwhelming dictates perceived pitch, but amplitude also does have some small, small effect on our pitch perception, especially when it is very large. For example, a very loud sound can have a different perceived pitch than you would predict from its frequency alone.
That all being said, usually these effects are negligible, and pitch can be thought of as equivalent to fundamental frequency.
Please explain elementarily; I'm never studied natural sciences.
I still don't accept intuitively how Amplitude can affect pitch. Can the bold sentences be exemplified by anything simple? The example in my mind is opera singers: a competent one's loud singing, shouldn't affect the frequency, because then (s)he'll sound off-tune.