This article is from the BiSexuality FAQ, by Jon Harley email@example.com with numerous contributions by others.
A strict definition of a bisexual would be someone who has romantic and/or
sexual relations with other people of more than one sex (though not
necessarily at the same time - see section A8).
However, since not everyone has necessarily had the opportunity to act on
their sexual/romantic attractions, some people prefer a looser definition; for
instance, that a bisexual is a person who - in their own estimation - feels
POTENTIALLY able to have such attraction. This could be anyone who has erotic,
affectionate, or romantic feelings for, fantasies of, and/or experiences with
both men and women.
A bisexual may be more attracted to one sex than the other, attracted equally
to both, or find people's sex unimportant (see section A7). The strength of
their attractions to men and women may vary over time.