An Etymological Dictionary of Astronomy and Astrophysics

General: The act of stretching or straining; the state of being stretched or strained.
Mechanics: The longitudinal deformation of an elastic body that results in its elongation.
Electricity: Voltage or potential; electromotive force.

Etymology (EN): M.E., from M.Fr. tension, from L. tensionem (nominative tensio) “a stretching,” from tensus, p.p. of tendere “to stretch,” cognate with Pers. taneš, as below.

Etymology (PE): Taneš, verbal noun from tanidan
“to spin, twist, weave;” Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to spin, stretch;" tanoti “stretches,” tantram “loom;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
PIE base *ten- “to stretch”), Pers. târ “string,” tân “thread,” tur “fishing net, net, snare,” and tâl “thread” (Borujerdi dialect) belong to this family; variants tanta “cobweb,” tanadu, tafen, kartané, kârtané, kâtené,
Pashtu tanistah “cobweb;” cf. Skt. tantu- “cobweb, thread, string.”