Unitary \YOO-ni-ter-ee\ , adjective;
1. Of or pertaining to a unit or units
2. Of, pertaining to, characterized by, or aiming toward unity
3. Government: Of or pertaining to a system of government in which the executive, legislative, and judicial powers of each state in a body of states are vested in a central authority
This word is first attested in 1816 from unit + -ary. Unit dates to the 1560's as "single number regarded as an undivided whole." It's an alteration of unity, which dates to the 1300's from Anglo-French unite, which derives from Old French unite. The Old French word comes from Latin unitatem ("oneness, sameness, agreement"), based on unus ("one") from Proto-Indo-European *oinos.