An Etymological Dictionary of Astronomy and Astrophysics

A state of balance or rest between the forces operating on or within a physical system. → stable equilibrium; → unstable equilibrium; → dynamical equilibrium.

Etymology (EN): From L. æquilibrium, from æquus, → equal + libra “a balance, scale.”

Etymology (PE): Tarâzmandi, noun of tarâzmand “in equilibrium,” from tarâz “level; a level” + possession suffix -mand. The first component
from tarâzu “balance, scales,” Mid.Pers. tarâzên-, taraênidan “to weigh;” Proto-Iranian *tarāz-, from *tarā- “balance, scale” (cf. Skt. tulā- “scales, balance, weight,” from tul- “to weigh, make equal in weight, equal,” tolayati “weighs, balances;” L. tollere “to raise;”
Gk. talanton “balance, weight,” Atlas “the Bearer” of Heaven;" Lith. tiltas “bridge;” PIE base telə- “to lift, weigh”) + Av. az- “to convey, conduct, drive,” azaiti drives" (cf. Skt. aj- “to dive, sling,” ájati “drives,” ajirá- “agile, quick;”
Gk. agein “to lead, guide, drive, carry off;” L. agere “to do, set in motion, drive,” from PIE root *ag- “to drive, move,” → act).