apostasy

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. Abandonment of one's religious faith, a political party, one's principles, or a cause.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. The renunciation of a belief or set of beliefs.
• n. Specifically, the renunciation of one's religion or faith.

from the GNU version of the Collaborative International Dictionary of English

• n. An abandonment of what one has voluntarily professed; a total desertion of departure from one's faith, principles, or party; esp., the renunciation of a religious faith.

from The Century Dictionary and Cyclopedia

• n. An abandonment of what one has professed; a total desertion of, or departure from, one's faith, principles, or party.
• n. In theology, a total abandonment of the Christian faith.
• n. In Rom. Cath. eccles. law: A persistent rejection of ecclesiastical authority by a member of the church. An abandonment without permission of the religious order of which one is a member. A renunciation of the clerical profession by one who has received major orders.
• n. In medicine, same as apostasis.
• n. Also spelled apostacy.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. the state of having rejected your religious beliefs or your political party or a cause (often in favor of opposing beliefs or causes)
• n. the act of abandoning a party for cause