Hypocrite

1. a person who pretends to have virtues, moral or religious beliefs, principles, etc., that he or she does not actually possess, esp. a person whose actions belie stated beliefs.
2. a person who feigns some desirable or publicly approved attitude, esp. one whose private life, opinions, or statements belie his or her public statements.

Hypocrisy (or the state of being a hypocrite) is the act of preaching a certain belief or way of life, but not, in fact, holding these same virtues oneself. For example, a teacher telling students they should not plagiarize, while secretly being a plagiarist himself. Hypocrisy is frequently invoked as an accusation in many contexts.