To be attracted to one/two OR MORE genders, and, if only to two, NOT ALWAYS JUST TO MALES AND FEMALES. The "bisexual umbrella" covers
pan/poly/etc.sexuality, so bisexual is basically just a broader term. Bisexuals are separated from and oppressed by the
straight and queer communities alike through
bi erasure by the term "straight passing privilege" (meaning that some bisexuals are in hetero relationships and, because they can "pass as
straight", don't deserve to go to pride or to be considered a part of the queer community. Because getting our identities erased is such a privilege). The attraction to two+ genders doesn't mean that we're sleeping with everyone, and being in a hetero/homo relationship doesn't make us hetero/homo, just like eating a
salad for
lunch doesn't make you a vegetarian. Basically, bisexuals are attracted to (usually, sometimes more) two genders.
Bisexuality is simply defined (by bisexuals) as attraction to more than one
gender. All groups of people have the right to
define themselves, and outsiders
will always try to impose their own definitions. In the end, it is the community itself who has final say on what it is.