(1):

(n.) The smallest particle of matter that can enter into combination; one of the elementary constituents of a molecule.

(2):

(n.) An ultimate indivisible particle of matter.

(3):

(n.) An ultimate particle of matter not necessarily indivisible; a molecule.

(4):

(n.) A constituent particle of matter, or a molecule supposed to be made up of subordinate particles.

(5):

(n.) Anything extremely small; a particle; a whit.

(6):

(v. t.) To reduce to atoms.