A homosexual is an individual that is sexually attracted to members of the same sex. For example, a homosexual male is attracted to males and a homosexual female is attracted to females. Homosexual males are often reffered to as "gay" and homosexual females are often reffered to as "lesbians."
No universally accepted definition exists within our contemporary society as to what constitutes homosexuality. Some believe that a person who has had sex with the member of one's own sex is homosexual, others - that a homosexual must prefer members of one's own sex, and yet others define it as the exclusive attraction to members of own sex.
In addition to that non-duality of genders makes the definition impossible. A transgendered person, for example, can keep one's own sexual orientation after the gender reassignment surgery, at which time the question arrises whether the term used to describe that orientation should change. And of course there are more than just two genders, which makes 'homo' (same) in 'homosexual' hard to pin down.