WordWeb Online

1. Not closed or secured
   "the car door was unfastened"; "unfastened seatbelts"
    
2. Affording unobstructed entrance and exit; not shut or closed
   - open
    
3. Not buttoned
   - unbuttoned
    
4. Not tied
   - untied

1. Cause to become undone
   "unfasten your belt"
    
2. Become undone or untied
   "The shoelaces unfastened"

See also: ajar, open, open-collared, unbarred, unbolted, undone, unfastener, unfastening, unlatched, unlocked, unsecured, wide-open

Type of: open, open up, undo

Antonym: fasten, fastened