1590s, "power to accomplish something," from Latin efficientia "efficient power; efficiency; influence" (from efficientem; see efficient) + -cy. In mechanics, "ratio of useful work done to energy expended," from 1858. Attested by 1951 as short for efficiency apartment (itself from 1917).

"power, greatness," also "grandeur," Old English mihtinesse; see mighty + -ness.

c. 1400, potentat, "a ruler, lord, prince, monarch; person who possesses independent power or sway," from Old French potentat and directly from Late Latin potentatus "a ruler," also "political power," from Latin potentatus "might, power, rule, dominion," from potentem (nominative potens) "powerful," from potis "powerful, able, capable; possible;" of persons, "better, preferable; chief, principal; strongest, foremost," from PIE root *poti- "powerful; lord." 

"potency, power, strength," early 15c., from Old French potence "power," from Latin potentia,from potis "powerful, able, capable," from PIE root *poti- "powerful; lord." The potence in watch-making, etc. (1670s) is from a special sense of French potence "a crutch."

late 14c., "power of sight," from stare (v.). From c. 1700 as "a fixed gaze."

"snobs collectively," especially as exercising social influence of power, 1838, from snob + -ocracy.

in France and the U.S., "the eighth power of 1,000" (1 followed by 24 zeroes); in the British system of numeration, "1 million to the seventh power" (1 followed by 42 zeroes), 1680s, from sept- (see septi-) + word-forming element abstracted from million. Compare billion.

"quality of being able, ability to do or act, power," Middle English might, from Old English miht, earlier mæht "bodily strength, power; authority, dominion, control; ability," from Proto-Germanic *makhti- (source also of Old Norse mattr, Old Frisian, Middle Dutch, Dutch macht, Old High German maht, German Macht, Gothic mahts), a Germanic suffixed form of the PIE root *magh- "to be able, have power."

"paralyzed; deprived of energy or power of action," 1540s, past-participle adjective from palsy.

"invested with, having, or bestowing full power," 1640s, from French plénipotentiaire and directly from Medieval Latin plenipotentiarius "having full power," from Late Latin plenipotens, from Latin plenus "complete, full" (from PIE root *pele- (1) "to fill") + potentem "powerful" (see potent). As a noun from 1650s, "person invested with full powers to transact any business," especially with reference to an ambassador to a foreign court or government, given full power to negotiate a treaty or transact other business.