at·om / ˈatəm/ • n. the basic unit of a chemical element. ∎  such particles as a source of nuclear energy: the power of the atom. ∎  an extremely small amount of a thing or quality: I shall not have one atom of strength left.

atom †body so small as to be incapable of division XVI; supposed ultimate particle of matter XVII; hence in mod. physics and chem. XIX. — (O)F. atome — L. atomus — Gr. átomos, sb. use of adj. ‘indivisible’, f. A-4 + *tom- (cf. -TOMY).
Hence atomic XVII. — modL.

atom A value that cannot be decomposed further. In LISP an atom is a representation of an arbitrary string of characters or the special atom NIL, i.e. nothing. The word is also used as a predicate in LISP-like languages to determine whether an arbitrary value is or is not an atom:

(atom (cons(h, t)))

always yields FALSE but

(atom, NIL)

and

(atom, “word”)

evaluate to TRUE.

atom (at-ŏm) n. the smallest constituent of an element that can take part in a chemical reaction. An atom consists of a positively charged nucleus with negatively charged electrons orbiting around it.