1. Physics: a.) the smallest component of an element having the chemical properties of the element, consisting of a nucleus containing combinations of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction; the number of protons determines the identity of the element. b.) an atom with one of the electrons replaced by some other particle: muonic atom; kaonic atom.
2. Energy: this component as the source of nuclear energy.
3. a hypothetical particle of matter so minute as to admit of no division.
4. anything extremely small; a minute quantity.