What is Apostacy?

Legal Definition
Eng. law. A total renunciation of the Christian religion, and differs from heresy. (q. v.) This offence is punished by the statute of 9 and 10 W. III. c. 32. Vide Christianity.
-- Bouviers Law Dictionary
Legal Definition
In English law. The total renunciation of Christianity, by embracing either a false religion or no religion at all. This offence can only take place in such as have once professed the Christian religion. 4 Bl. Comm. 48; 4 Steph. Comm. 231.
-- Black's Law Dictionary