Chastity, or purity, is opposite of the deadly sin, lust. The Wikipedia definition is:
“sexual behavior of a man or woman acceptable to the ethical norms and guidelines of a certain culture, civilization or religion.”
Pretty good definition if you ask me. I hate to get all Bill Clinton-esque, but it does beg the question of what exactly “acceptable” means to that certain culture or civilization. The accepted sexual behavior of someone living as a Christian is drastically different than that of someone living as a Jew, Bhudist, Hindu, …