Gay
From YGSWiki
See also the Wikipedia article on homosexuality.
Gay generally means someone who is attracted exclusively to people of the same sex or gender as themselves.
Today, referring to people as "homosexual" tends to be frowned upon, and when done in conversation, is taken as an indication that the speaker is uncomfortable and/or very unfamiliar with discussing gay people or issues. It's generally seen as an awkward clinical term, whereas gay is a term that comes from within the community and is far less of a mouthful to say!
"Gay" can also be used to reference certain behaviours that are stereotypical of gay people, especially gay males, or to reference behaviours or things that are associated with gay culture.
