The condition of having reproductive organs of one sex only.
2rare The condition of having members of one sex predominant.
3rare The condition of having sexual relations in which one seeks to gratify only oneself.
4The state or condition of being sexually attracted to members of one sex only. Opposed to bisexuality.
Early 20th century; earliest use found in Abraham Arden Brill (b. 1874). From mono- + sexuality, after monosexual.