Homosexuality can refer to both attraction or sexual behavior between people of the same sex, or to a sexual orientation.
When describing the latter, it refers to enduring sexual and romantic attraction towards those of the same sex, but not necessarily to sexual behavior.
Homosexuality is contrasted with heterosexuality, bisexuality and asexuality.
Lesbian denotes a homosexual woman.
Most scientist today agree that sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors.
Although homosexuality does not appear to be adaptive from an evolutionary standpoint, because homosexual sex does not produce children, there is evidence of its existence through human history.
Although a number of biological factors have been considered by scientists, such as prenatal hormones, chromosomes, polygenetic effects, brain structure and viral influences, no scientific consensus exists as to how biology influences sexual orientation.
Most scientists agree that it is unlikely that there is a single "gay gene" that determines something as complex as sexual orientation, and that it is more likely to be the result of an interaction of genetic, biological and environmental/cultural factors.