In a general sense, the practice of restraining oneself from indulging in something; for example, alcohol. However, it has become commonly associated with the abstaining of sex – and in Queer lexicon, typically refers to the conscious decision not to engage in same-sex relations, usually because a religious, cultural or political belief forbids it.
For example, in many conservative, homophobic religions, they may insist that same-sex attracted people abstain from sexual relations to abide by their religion. Abstinence is usually part of the course in “conversion therapy“.
This should not be confused with asexuality, which is not a conscious choice, but a lack of sexual desire.