A Jewish word that means a person who is not of the Jewish faith. If you are of the faith, and choose to marry a shiksa, you will traditionally be considered 'dead' (Having married out) and your relatives will be in shiva, a period of mourning following someone's death.

It's specifically a non-Jewish woman; the male form is sheygetz. It's not a clinical or legal term: it's very definitely derogatory.