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 stat

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.