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)
2. A small, narrow opening; a loophole. And stop all sight-holes, every loop from whence The eye of reason may pry in upon us. (Shak)
3. A curve of any kind in the form of a loop.
4. A wire forming part of a main circuit and returning to the point from which it starts.
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.
Origin: cf. Ir. & gael. Lub loop, noose, fold, thong, bend, lub to bend, incline.