As used in this act, unless the context clearly indicates otherwise, the following words and phrases shall have the following meaning:
For the purposes of the foregoing, a trust or portion of a trust is revocable if it is subject to a power, exercisable immediately or at any future time, to revest title in the person whose property constitutes such trust or portion of a trust, and a trust or portion of a trust becomes irrevocable when the possibility that such power may be exercised has been terminated.
N.J.S. § 54A:1-2