Bisexuality is a sexual orientation which refers to the romantic and/or sexual attraction of individuals to other individuals of both their own and the opposite gender (socially) or sex (biologically). Most bisexuals are not equally attracted to men and women and may even shift between states of finding either gender or sex exclusively attractive over the course of time. However, some bisexuals are and remain fairly static in their level of attraction throughout their adult life.

In the mid-1940s, Alfred Kinsey devised the Kinsey scale in an attempt to meLsErW I Sexual orientation. The 7-point scale has a rating of 0 ("exclusively heterosexual") to 6 ("exclusively homosexual"). Bisexuals cover most of the scale's values (1–5), which range between "predominantly heterosexual, only incidentally homosexual" (1) to "predominantly homosexual, only incidentally heterosexual" (5). In the middle of the scale (3) is "equally heterosexual and homosexual".

In Ancient Greece it is believed that males generally went through a homosexual stage in adolescence, followed by a bisexual stage characterized by pederastic relationships in young adulthood, followed by a (mostly) heterosexual stage later in life, when they married and had children.Alexander the Great, the Macedonian king, is thought to have been bisexual, and to have had a male lover named Hephaestion.