Bisexual. Almost always used as an adjective (unlike "homosexual" and "transsexual", which are often incorrectly used as nouns), but quite often used as a noun in plural ("bisexuals"). Quite often shortened to "bi" as a slang term. "Bisexual" does not mean Hermaphrodite. It also does not mean Transgender. "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 the same gender and other genders. Note that this does not mean that bisexuals must be polyamorous, with partners of more than one gender, 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.
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 more than one.
As can be seen from the above conflation of bisexuality with polyamory, attraction to multiple 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 multiple sexes and are only pretending to be for the attention. Sadly, to the annoyance of actual bisexuals everywhere, this one is true in some cases.
Additionally, being bisexual does not necessarily require equal attraction to multiple 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.
- X — Non-sexual, no attraction to any sex.
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 or 5 on the Kinsey scale are sometimes referred to with the neologisms "bi-curious" and "homoflexible"/"heteroflexible" as they may engage in bisexual relations for experimentation.
People that are X on the scale are asexual, they do not feel sexual attraction towards any sex but they can experience romantic attraction. This is also applicable for bisexual people, terms such as heteroromantic, homoromantic, biromantic, and aromantic exist because of the distinction between romantic and sexual orientation. People that are sexually attracted to multiple sexes but are romantically attracted to only one or none would thus be described as heteroromantic bisexuals, homoromantic bisexuals or aromantic bisexuals under such terminology, as opposed to biromantic bisexuals. A biromantic asexual may also consider themselves as "bi".
Something else that probably needs to be said: while bisexuals find both men and women attractive, they do not find all people 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 people of more than one sex; 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). The term "polysexual" (not to be confused with polyamory) also exists, which is when people are attracted to more than one but not all genders (implying that there are more than two), although this one isn't used very often and does not specify which genders the person is attracted to.
Despite the debate on terminology, however, the definition of bisexuality used by all major bisexual organizations is "attraction to the same and other genders," not excluding people of non-binary genders. "Bisexual" is therefore often used as an umbrella term to refer to any number of non-monosexual identities. Although this is still highly problematic as the term itself originated within the binary framework to compliment the terms "heterosexuality" and "homosexuality"; the general public continues to view the orientation with its original meaning and most self-identified bisexuals only feel attraction towards the binary male and female genders. The polysexual and pansexual communities were first created to distinguish themselves from bisexuality and many of them most certainly do not appreciate bisexuality being re-defined to take up their spaces and having their orientation identities erased. Especially since they were the original creators of terms that laid out of the binary not bisexuals and they would prefer being called "multisexual" rather than being swept under the bisexual rug.
In fact, this confusion has lead to the development of another term used to describe bisexuality called, "ambiphilia". "Androphilia" and "gynephilia" are terms used in behavioural science to describe sexual orientation, as an alternative to a gender binary homosexual and heterosexual conceptualization. This system is much more grounded in science and is more commonly used for studies than for personal labels, but it they are still applicable and useful. Androphilia describes sexual attraction to masculinity; gynephilia describes the sexual attraction to femininity and ambiphilia describes people that are attracted to both masculinity and femininity. According to this definition, bisexuality is not binarist as it does not imply there are only two genders. It is the attraction towards masculine and feminine gender expression and gender identity is not a necessary component.
What constitutes as "bisexuality" is probably as debatable as the idea of what asexuality is. It was not originally thought of as a spectrum orientation, but is now seen as one by many people due to the stretching of the term in recent years by asexual organisations. The inclusion of the disputable "demisexual" orientation within asexuality has caused outrage from many people. These examples of identity politics display the complexity with labelling systems and defining sexual orientation. The term "bisexual" is a highly contentious issue that means many different things to many different people. Labels are made to help people feel secure, not confuse them. Although simplifying something as multiplexed as gender identity and sexual orientation with our current labelling systems might not be the best thing, either. In the end, people should choose whatever they think is best or right for them.
See Bisexuality Tropes here.