In its simplest definition, sexual orientation is the emotional and/or physical attraction between two individuals; this attraction can be between two people of the same sex, of different sex, or between an individual and another person of either sex. When one's sexual orientation is heterosexual, one is said to be attracted to individuals of the opposite sex; when one's sexual orientation is homosexual, one is said to be attracted to individuals of the same sex. If an individual is attracted to individuals of either sex, they are said to be bisexual.
However, the concept of gender also needs to be taken into consideration as well. Someone whose physical biology defines them as a male might not identify as such; pre-operative transsexuals and transgendered individuals might have the genetic makeup of a male but live their lives as a female. While many would argue that sexual orientation is rooted strictly in genetics and biology, taking into consideration these unique circumstances can make such a definition inaccurate. A transgendered woman, that is an individual with male biology but who identifies as a woman, that is attracted to women would likely consider herself a lesbian; though her biology would dictate that she is engaged in a heterosexual relationship, as she is biologically male and is attracted to a female, it is her gender identity that makes the determination of her sexual orientation.