Some say its anyone who leaves the WT, others say its anyone who actively works against the org. Many talks say active dubs can be apostates if they're critical of anything in the Borg.
In a JW's mind, an apostate is a vile person. The very word activates an irrational fear of that person's cult persona. For instance, my wife refuses to acknowledge that I'm an apostate. She is still unable to put a real person to the title 'apostate.' In her mind, the word 'apostate' is linked with 'demonized.'
The WT has built up such a phobia with talks such as the one we had at the DC this year. This is mind control at its finest. Their brains are inoculated from anything someone they see as an apostate says. They have no problem calling them names, treating them like dirt and saying they would kill them if they were allowed. What the WT has done is strikingly similar to what the nazis did to the Jews. If we are dehumanized no one will care what we have to say, and will consider us like animals. Like we're not real people.