# What is a “thermal” particle?

I have seen various uses of the word "thermal" recently (e.g. in black hole physics) that make me wonder what is the exact meaning of "thermal" in particle physics. There are also "thermal neutrons", for example, so my initial understanding of "thermal" as related "thermal radiation", i.e. propagated by photons, seems inadequate.

Can I apply "thermal" to any particle which meets the following criterion?

A thermal particle is a free particle with a kinetic energy corresponding to the most probable velocity at a temperature of 290 K (17 °C or 62 °F), i.e. the mode of the Maxwell–Boltzmann distribution for this temperature.

Or should the criterion be:

A thermal particle is a free particle with a kinetic energy corresponding to the most probable velocity at the temperature of its environment, i.e. the mode of the Maxwell–Boltzmann distribution for this temperature.

• Your question is too broad to be answered. This question should be closed, in my opinion. – Yashas Feb 11 '17 at 6:08
• @YashasSamaga - made the question narrower. – Frank Feb 11 '17 at 7:07