From the Reasoning from the Scriptures book, pg. 34:
Definition: Apostasy is abandoning or deserting the worship and service of God, actually a rebellion against Jehovah God. Some apostates profess to know and serve God but reject teachings or requirements set out in His word. Others claim to believe the Bible but reject Jehovah's organization.
This organization would of course be the one which has been carefully crafted by celebrated WT scholars.