Etymology
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potency (n.)

"power, inherent strength, ability to accomplish or effect," mid-15c., potencie, from Latin potentia "power," from potentem "potent," from potis "powerful, able, capable," from PIE root *poti- "powerful; lord." 

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*poti- 

Proto-Indo-European root meaning "powerful; lord." 

It forms all or part of: bashaw; compos mentis; despot; hospodar; host (n.1) "person who receives guests;" idempotent; impotent; omnipotent; pasha; plenipotentiary; posse; possess; possible; potence; potency; potent; potentate; potential; potentiate; potentiometer; power; totipotent

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit patih "master, husband;" Greek posis, Lithuanian patis "husband;" Latin potis "powerful, able, capable; possible." 

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pH 
1909, from German PH, introduced by S.P.L. Sörensen, from P, for German Potenz "potency, power" + H, symbol for the hydrogen ion that determines acidity or alkalinity.
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potence (n.)

"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."

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impotence (n.)
early 15c., "physical weakness," also "poverty," from Old French impotence "weakness" (13c.), from Latin impotentia "lack of control or power," from impotentem "lacking control, powerless" (see impotent). In reference to a complete want of (male) sexual potency, from c. 1500. The figurative senses of the word in Latin were "violence, fury, unbridled passion," via the notion of "want of self-restraining power," and these sometimes were used in English. Related: Impotency.
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virtual (adj.)
late 14c., "influencing by physical virtues or capabilities, effective with respect to inherent natural qualities," from Medieval Latin virtualis, from Latin virtus "excellence, potency, efficacy," literally "manliness, manhood" (see virtue). The meaning "being something in essence or effect, though not actually or in fact" is from mid-15c., probably via sense of "capable of producing a certain effect" (early 15c.). Computer sense of "not physically existing but made to appear by software" is attested from 1959.
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