Baal teshuva or ba'al teshuvah–בעל תשובה‎ (male) בעלת תשובה(female)–is a term that often refers to a Jew who turns to embrace Orthodox Judaism.

Baal teshuva or ba'al teshuvah (Hebrew: בעל תשובה‎; for a woman, בעלת תשובה, baalat teshuva; plural, בעלי תשובה, baalei teshuva), sometimes abbreviated to BT, is a term that often refers to a Jew who turns to embrace Orthodox Judaism. Baal teshuvah literally means, "master of repentance" or "master of return" i.e., one who has repented or "returned" to God. It is often contrasted with "FFB" (Frum from birth), which refers to Jews who are born into families that are already religiously observant, and who have been conceived, born, and raised Jewishly.

Historically, the term referred to a Jew who transgressed the halakhah (Jewish law) and completed a process of introspection to "return" to the straight path. In contemporary times, the phrase is primarily used to refer to a Jew from a secular background who becomes religiously observant (normally in an Orthodox fashion) later in life. (The alternative term, chozer biteshuvah (חוזר בתשובה) is more commonly used in Israel).

Source: http://en.wikipedia.org/wiki/Baal_teshuva