It changes so much. But from what my understanding is, a Apostate in JW world is a baptized JW that rejects any of the current teachings and then talks to others about it.
JW will automatically claim any source as apostate unless it comes from their demi-gods the Governing Body of JW. This is especially true for any negative comment about their teachings or actions.
If a witness was to have a conversation with a group of Elders, it is more than likely the Elders wouldn't even allow you to present any material from outside the Watch Tower for that conversation. I've seen this first hand and also on some secretly taped Judicial meetings.
But from my understanding the term apostate basically means someone that rejects their faith. But in JW world it means a lot more. You are basically evil and at one time I remember it was taught you committed the unforgivable sin and their is no redemption for you. Not sure if that is the thinking/teaching now.
They adjust their teachings all the time.