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 or sex. Most bisexuals are not equally attracted to men and women and may even shift between states of finding either 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-1950s, Alfred Kinsey devised the Kinsey scale in an attempt to measure 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".
Although observed in a variety of forms in human societies and in the animal kingdom throughout recorded history, the term bisexuality (like the terms hetero- and homosexuality) was only coined in the 19th century.