Unary

Unary \YOO-nuh-ree\ , adjective;
1. Composed of a single item or element

This word is first attested in 1923 as a chemistry term. It's part of a series of numerical descriptors similar to single, double, triple, etc.. This series is unary, binary, tenary, etc..  It seems that this -ary series is something of a back-formation from ternary and binary, which both pre-date unary by centuries.  

Ternary is first attested in 1430 as an adjective meaning "pertaining to, consisting of, compounded of, or characterized by a set (or sets) or three." It comes from Late Latin ternarius ("consisting of three").

Binary is first attested in 1464 as a noun meaning "a combination of two things; a couple." It's earliest use as an adjective dates to the late 16th century and was a musical term, as in binary measure ("that which has to beats to a bar"). Binary as we, the computer crowd, know it dates to 1948.