1. A fold or doubling of a thread, cord, rope, etc, through which another thread, cord, etc, can be passed, or which a hook can be hooked into; an eye, as of metal; a staple; a noose; a bight. That the probation bear no hinge, nor loop to hang a doubt on. (Shak)
5. (Science: physics) The portion of a vibrating string, air column, etc, between two nodes; called also ventral segment. Loop knot, a single knot tied in a doubled cord, etc. So as to leave a loop beyond the knot.