If you look up "apostasy" on Wikipedia, and scroll down, you will see a separate subsection for Jehovah's Witnesses.
In dictionaries, the word is usually defined along the lines of a renunciation of one's reliigion (or principles or cause etc), but most Christians, if they use it at all, tend to restrict the term to those who abandon faith in God altogether, not those who change religious denomination, or those who dispute individual teachings or leaders within a denomination.