However, many members of bisexual communities tend to prefer the definition:
'a changeable sexual and emotional attraction to people, where gender may not be a defining factor'.
It is extremely difficult to estimate the extent of bisexuality because it depends very much on how the term is defined. For example, it could be people who identify themselves as bisexual (in which case the estimate might be rather small), or it could be all people who have ever had an aesthetic, romantic or sexual attraction to more than one gender (in which case the estimate might be rather large).
In the 1940s, Alfred Kinsey put forward his famous scale of sexual orientation, finding that many people did not fall simply at either end of the spectrum (heterosexual or homosexual).
In terms of the proportion of people who are actively involved in UK bisexual communities, over two hundred people participate in the annual BiCon event - just a small proportion of the bisexual-identified people who are involved in local and on-line communities. Fears of biphobia may prevent some people from being 'out' about their bisexuality.
A common stereotype of bisexuality is that it is 'a phase' on the way to a 'mature' lesbian, gay or straight identity. Some recent research has even attempted to prove the non-existence of bisexuality, particularly male bisexuality, although these studies have been criticised as methodologically and theoretically flawed. Bisexual women are frequently regarded as 'just being bi-curious' and trying to titillate heterosexual men: another way of denying that bisexuality is 'real'.
There is also a common stereotype that bisexuals are greedy and promiscuous. This can lead to a double bind for bisexuals whereby those who are in non-monogamous relationships are regarded as proving this stereotype (even if these are honest open relationships), whereas those who are single or in monogamous relationships are regarded as 'really' lesbian, gay or straight and risk invisibility.
Bi invisibility is also perpetuated in the media when celebrities and fictional characters are portrayed as lesbian or gay even though they have sexual/romantic relationships with women and men.
The author Robyn Ochs writes about the 'double discrimination' bisexual people can face from both heterosexual and lesbian/gay communities. Many surveys have found that bisexual people suffer from higher rates of mental health problems than lesbians and gay men, who in turn have higher rates than the population as a whole. This is often linked to biphobia, bisexual invisibility, low levels of support and acceptance, and the 'double discrimination' experienced by bisexual people.
You can also read Stonewall's guide Bisexual People in the Workplace which outlines practical advice for employers on how to make work environments better for their bisexual staff.