Excellent article on a question I’ve had for a long time.

“In the broadest sense, ‘particles’ are physical things that we can count,” says Greg Gbur, a science writer and physicist at the University of North Carolina in Charlotte. You can’t have half a quark or one-third of an electron. And all particles of a given type are precisely identical to each other: they don’t come in various colors or have little license plates that distinguish them. Any two electrons will produce the same result in a detector, and that’s what makes them fundamental: They don’t come in a variety pack.

Source: What is a “particle”?