Officially, Watchtower defines an apostate as a baptized JW who rejects bible teachings as taught by Jehovah's Witnesses. So a JW does not have to voluntarily leave the organization to be an apostate. He can be disfellowshipped for apostasy if adamantly disagrees with Watchtower teachings.
Since JWs teach that a person must worship God as part of their organization to have God's favor, the word apostate also applies to any who disassociates from the organization - even without them being openly critical of its teachings or campaigning against it.
That's the official position. Unofficially, however, most JWs use the word apostate to refer only to JWs who become critics of the organization and its teachings and actively campaign to influence JWs into leaving the organization. They also use the word apostate as an adjective to refer to information that is critical of their teachings and organization.
The true dictionary definition of the word apostate means someone who has abandoned or rejected a religion or cause they once supported. When JWs start talking about apostates you can ask the JW what religion he was before he became a JW and why he left that religion. Then you tell the JW: "You're an apostate of [insert religion]! Why should I believe anything you have to say about them?"