The sexual attraction to both genders, without being partial to just one. To engage in sex but also to have similar emotional attachments with both genders.
Having a sexual interest and/or emotional attachment to a person, irrespective of their gender and/or sexual orientation. In many instances, it simply refers to a person being open to such involvements, irrespective of the gender. It DOES NOT IMPLY any actual action taken.
This can include a physical attraction to a person of the same gender, in much the same way that one is attracted to a person of the opposite gender.
There are many 'variables' involved in what makes a person bisexual.
Using the Kinsey Scale, it is determined that many who identify having some 'same sex attractions' or who might 'engage in same sex' are not actually classifying themselves as Bisexual, due to the ambiguity of the definition.
Ancient times seem to show a great deal of such behavior, in its culture. Only the advent of modern religion has there been a turning away from what most ancient cultures considered a requirement, such as in the practice of pederasty in the ancient civilizations.
Roman Emperor Hadrian for example, fell in love with Antinous, and actually made him a God after the death of the younger man.
Spartan Law for example, insisted that older men take younger men and have sex with them, to bring them into manhood. The law did require or stipulate, that it was required, only if later on there was a marriage with a member of the opposite sex, and that children were born.
The relationship supposedly did continue beyond childhood, but to what degree is unclear. In some instances, those who did continue their relationships with their mentor, were looked down upon.
The advent of Modern Religion (Christianity, Judaism, Islam) has defined the line between the sexes which has led to many people to ignore their true sexual orientation. This includes bisexual persons, who believe crossing the gender line to be a sin, or in violation of some higher power's will.
History (Legend): Some claim that Alexander the Great was bisexual, and that he had a male lover. There are also some who claim he was homosexual.
As varied as sex, the practice is complex. It can be simply the attraction of another person of the same gender, of admiring their appearance, or specific features.
Females tend to be more open in this, and act more upon it, perhaps, than their male counterparts.
Physical appearances can elicit a sexual response with a person, even though the other party is of the same sex. Whether it goes further or not is not necessarily determined one's sexuality either.
Sex between bisexuals of the same gender can be simply a physical attraction, or based more on an emotional attraction. The actual sex act, does not mean that the bisexual actually favors it, or prefers it, or that it is even equal to other sexual acts with different genders.
Practice (Associated Acts): Bisexuality is not an equal preference for both sexes, or genders. It can be where a male may find certain attributes of another desirable, admirable, and even become aroused by it, but the concept of engaging in actual sex is not acceptable. The same holds true for females.
There are degrees of bisexuality, and many will dispute the term even, as it is too confining. Uses of other terms such as omnisexual, which refers to the person's willingness to 'consider' alternative affections that are not based on gender.
Noteworthy: While in the Kinsey report, the claim that as many as 47% of adult males have thoughts of other males, it does not state to what degree such thoughts become acted upon.
The terms Heterosexual, Homosexual, and Bisexual are all modern terms, from the 19th Century.
Many other terms have been developed, to address the differences between the entire Bisexual Community. An example being, that if one does not actively pursue members of the same sex, but are open to such advances, they are referred to as being 'bi permissive' rather than bisexual.
Some in the Gay Community will consider a 'bisexual' person to simply be one who has yet to come to terms with their true sexuality, implying they are either heterosexual or homosexual.
Others will claim it as being a 'cover' as bisexuality is less despised in today's society.