A person (man or woman; the common myth being women are more likely to be bisexual than men) who can be attracted sexually to people of both genders. While it is true that some people will experience a period of bisexuality in the transition from hetero to homosexuality (or back to heterosexuality), bisexuals are not usually in this 'phase' but in it for the long haul.
Some bisexuals will view themselves as being eager for fun with either gender; others will tell you they just dont mind and have no preference, or aren't bothered by encounters with either gender (not to be confused with those mythical people who change their sexuality because they can't get any, or are just a superslut/superwhore - altho some bisexuals are superwhores, but so are straight and gay people)
Some bisexuals may be attracted to any gender at any time; some may consciously opt for men, women, or both at different times depending on their mood (ie on a night out a bisexual may aim to hook up with a man)
I'm bisexual, and im one of the latter ones of the above sentance.