Consisting of, or containing, one atom; as, the molecule of mercury is monatomic.

Having the equivalence or replacing power of an atom of hydrogen; univalent; as, the methyl radical is monatomic.

of or relating to an element consisting of a single atom; "helium and argon are monatomic gases"

A term describing an element that exists as single atoms. This in contrast to diatomic elements.

In physics and chemistry, monatomic is a combination of the words "mono" and "atomic," and means "single atom." It is usually applied to gases: a monatomic gas is one in which atoms are not bound to each other.