Tower Controller described an inadvertent release of an IFR aircraft when the TRACON's 'hold for release' was interpreted as released; the reporter suggested a phraseology change.

Narrative

I called TRACON for two departure releases. The first was a CJ1 departing Runway XX on the SID which is a left downwind departure and was released. I then requested release on the BE9; Runway XX second departure which is straight out to the northwest. I said we would provide visual separation between him and the CJ1. As I was saying the last part of my request the TRACON Controller started to say 'Hold for Release.' As I finished my request; all I heard from the TRACON Controller was 'Release;' and his initials. I gave my initials and coordinated the visual separation and release with the Local Controller; who then cleared the BE9 for takeoff. A couple minutes later; the Controller at the neighboring airport called to release the BE9; but the Ground Controller said he was already airborne. Order 7110.65; refers to the phraseology 'Hold for Release.' Better phraseology and a better operating practice is to either use the word 'HOLD' or 'RELEASED.' That phraseology would have made this a non-event; because you cannot mistake the word 'HOLD' for 'RELEASED;' but it is certainly possible for a Controller to say 'Hold For Release;' and the other Controller to only hear released. That has happened many times at this facility alone. Granted; I need to listen better; or repeat the instructions back to the originating Controller. The other Controller should not interrupt; because important information can be missed; as in this case.