loop

Etymology

From Middle English loupe (“noose, loop”), earlier lowp-knot (“loop-knot”), of North Germanic origin, from Old Norse hlaup (“a run”), used in the sense of a "running knot", from hlaupa (“to leap”), ultimately from Proto-Germanic *hlaupaną (“to leap, run”). Compare Swedish löp-knut (“loop-knot”), Danish løb-knude (“a running knot”), Danish løb (“a course”). More at leap. The verb is derived from the noun.

noun

1. A length of thread, line or rope that is doubled over to make an opening.
2. The opening so formed.
3. A shape produced by a curve that bends around and crosses itself.
   Arches, loops, and whorls are patterns found in fingerprints.
4. A ring road or beltway.
5. An endless strip of tape or film allowing continuous repetition.
6. A complete circuit for an electric current.
7. (programming) A programmed sequence of instructions that is repeated until or while a particular condition is satisfied.
8. (graph theory) An edge that begins and ends on the same vertex.
9. (topology) A path that starts and ends at the same point.
10. (transport) A bus or rail route, walking route, etc. that starts and ends at the same point.
11. (rail transport) A place at a terminus where trains or trams can turn round and go back the other way without having to reverse; a balloon loop, turning loop, or reversing loop.
    In 1908 the line was extended to a station called Wood Lane, which was built on a terminal track loop so that trains could turn round and go back the other way, … 2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, page 119
12. (algebra) A quasigroup with an identity element.
13. A loop-shaped intrauterine device.
14. An aerobatic maneuver in which an aircraft flies a circular path in a vertical plane.
15. A small, narrow opening; a loophole.
16. Alternative form of loup (“mass of iron”)
17. (biochemistry) A flexible region in a protein's secondary structure.

verb

1. (transitive) To form something into a loop.
2. (transitive) To fasten or encircle something with a loop.
3. (transitive) To fly an aircraft in a loop.
4. (transitive) To move something in a loop.
5. (transitive) To join electrical components to complete a circuit.
6. (transitive) To duplicate the route of a pipeline.
7. (transitive) To create an error in a computer program so that it runs in an endless loop and the computer freezes up.
8. (intransitive) To form a loop.
9. (intransitive) To move in a loop.
   The program loops until the user presses a key.
   The outstanding Tom Palmer won a line-out and then charged into the heart of the Welsh defence, scrum-half Ben Youngs moved the ball swiftly right and Cueto's looping pass saw Ashton benefit from a huge overlap to again run in untouched. February 4, 2011, Gareth Roberts, “Wales 19-26 England”, in BBC
10. To place in a loop.
    It found that trains often looped on their journey emit 14% to 20% more NOx and particulates than non-stop services. January 13 2021, Richard Clinnick, “Longer freight trains boost efficiency and reduce carbon”, in Rail, page 10