Particle (noun)

1. A tiny piece of matter.
2. A basic unit of a physical system, such as an atom or molecule.
3. A word or morpheme indicating grammatical distinctions such as tense, mood, or case.

Origin:

Late middle english: from latin particula "small part", diminutive of pars, part-.

Examples:

1. Scientists are studying the properties of subatomic particles.
2. The particle accelerator smashed atoms together at high speeds.
3. The phrase "have been" consists of two particles, "have" and "been".
4. Some particle in the air must have irritated his eyes.
5. He was studying the behavior of particles in a fluid.