Bisexual
Bisexual. Almost always used as an adjective (unlike "homosexual" and "transsexual", which are often used as nouns), but quite often used as a noun in plural ("bisexuals"). Quite often shortened to "bi" as a slang term.
Let's clear this up now: "bisexual" does not mean Hermaphrodite. It also does not mean Transsexualism. "Bisexual" is a term of sexual orientation, not gender identity.
The actual definition: unlike heterosexuals and homosexuals, who are attracted to just people of the opposite sex and just people of the same sex respectively, bisexuals are attracted to people of both sexes. Note that this does not mean that bisexuals must be polyamorous, with a male partner and a female partner at any given time, any more than somebody who is attracted to people of all hair colours would need to be polyamorous with a partner of each hair colour. Bisexuals don't separate attraction to each sex into two separate drives, but rather their sexual attraction is simply directed towards both halves of the population rather than just one.
Similarly, a bisexual person with an opposite-sex partner has not "turned straight", and a bisexual person with a same-sex partner has not "turned gay". It is how people feel that determines their sexuality, and not what they do. Even somebody who has only had sex with people of one gender still counts as bisexual if they feel attracted to both.
As can be seen from the above conflation of bisexuality with polyamory, attraction to both sexes can be a confusing idea for people who aren't bisexual, given the human tendency to prefer clearly demarcated boundaries and opposites -- black and white, up and down, male and female, gay and straight. This has lead to a fairly widespread belief that bisexuality doesn't exist, and that bisexuals are either deluded heterosexuals or are homosexuals with "one foot in the closet". Bisexuals, naturally, say that this is absolute crap. Another charge often laid at the feet of bisexuals is that many of them aren't really attracted to both sexes and are only pretending to be for the attention. Sadly, to the annoyance of actual bisexuals everywhere, this one has some truth to it.
Additionally, being bisexual does not necessarily require equal attraction to both sexes, with no particular preference. There are varying degrees of bisexuality ranging from mostly-straight to mostly-gay, as can be seen in the Kinsey Scale below:
- 0 -- Attracted to the opposite sex only, and not to the same sex at all.
- 1 -- Attracted to the opposite sex mostly, with some attraction to the same sex.
- 2 -- Attracted to both sexes, with a general preference towards the opposite sex.
- 3 -- Attracted to both sexes, with no particular preference for one over the other.
- 4 -- Attracted to both sexes, with a general preference towards the same sex.
- 5 -- Attracted to the same sex mostly, with some attraction to the opposite sex.
- 6 -- Attracted to the same sex only, and not to the opposite sex at all.
0 is heterosexuality. 6 is homosexuality. 1-5 are all bisexuality. In fact, some people believe that "true" heterosexuals and homosexuals are much rarer than people think, and that most people are bisexual to at least some degree -- manifestations of this being the tropes Even the Guys Want Him, Even the Girls Want Her and If It's You It's Okay. People who are at 1 on the Kinsey scale are sometimes referred to with the neologisms "bicurious" and "heteroflexible".
Something else that probably needs to be said: while bisexuals find both men and women attractive, they do not find all men and women attractive (any more than straight people find everyone of the opposite sex attractive, for example) and do not as a rule want to screw Anything That Moves. Bisexuals are attracted to men and to women; not to children, animals or inanimate objects. This is a particular stereotype that really pisses bi people off.
Some people don't like the term "bisexual" because of its apparent implication that people can only be one of two genders (as the "bi" prefix literally means "two", like bicycle and binoculars). There are many people who identify as being not male or female but somewhere in between (genderqueer), or something entirely different, or even without gender at all (agendered). Additionally, there are intersex people who are biologically somewhere in between male and female. Consequently, many people instead prefer the term "pansexual" -- which is intended to indicate an attraction to all genders (as opposed to both genders).
See Bisexuality Tropes here.