[elbasti]

This might sound tautological but a particle is, well, a thing that behaves like a particle.

Those behaviors are something like:

- it has momentum - it's state is uniquely defined by a position in space and a velocity

What's not a particle? A wave (well, until 1900 or so ...).

Sort of like asking "what is a number?"

A number is a thing that obeys certain rules. (You can add them; there's an `identity `, for every number there's a number which if you add together gives zero, etc).

That allows things like `(3 + 5i)` to be a number, for example.

[mr_toad]

> a particle is, well, a thing that behaves like a particle

Except when they don’t

https://en.wikipedia.org/wiki/Electron_diffraction