An Etymological Dictionary of Astronomy and Astrophysics

General: Anything shaped more or less like a loop, i.e. portion of a cord, ribbon, etc., folded or doubled upon itself.
Physics: The part of a vibrating string, column of air or other medium, etc., between two adjacent nodes.
Computers: The reiteration of a set of instructions in a routine or program.

Etymology (EN): Probably of Celtic origin (cf. Gael. lub “bend,” Ir. lubiam), influenced by O.N. hlaup “a leap, run.”

Etymology (PE): Gerdâl, from gerd “round, a circle”
(Mid.Pers. girdag “disk, round,” from gird/girt “round, all around,” Proto-Iranian *gart- “to twist, to wreathe,” cf. Skt krt “to twist threads, spin; to wind; to surround;” kata- “a twist of straw,” Pali kata- “ring, bracelet,” Gk. kartalos “a kind of basket,” kyrtos “curved”) + → -al.