at|om

at|om «AT uhm», noun.

1. the smallest particle of a chemical element that can take part in a chemical reaction without being permanently changed. An atom is made up of protons and neutrons in a central nucleus surrounded by electrons. Nearly all of its mass is concentrated in this nucleus. A molecule of water consists of two atoms of hydrogen and one atom of oxygen. »

About one hundred million atoms laid side by side are an inch long (J. L. Crammer and R. E. Peierls).

2. Figurative. a very small particle; tiny bit: »

There is not an atom of truth in the whole story.

SYNONYM(S): iota, jot, grain, tittle.

3. one of the ultimate particles of matter, according to ancient philosophy, which came together to form the universe.

╂[< Middle French atome, learned borrowing from Latin atomus < Greek átomos indivisible < a- without + tómos a cutting]