Etymology
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capacity (n.)

early 15c., "ability to contain; size, extent;" also "ability" in a legal, moral, or intellectual sense, from Old French capacité "ability to hold" (15c.), from Latin capacitatem (nominative capacitas) "breadth, capacity, capability of holding much," noun of state from capax (genitive capacis) "able to hold much," from capere "to take" (from PIE root *kap- "to grasp").

Sense of "power to store electricity" is from 1777; industrial sense of "ability to produce" is from 1931. Meaning "power of containing a certain quantity" is from 1885, hence "largest audience a place can hold" (1908).

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capacitance (n.)
"ability to store an electric charge," 1893, from capacity + -ance.
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capacitor (n.)
"device which stores electricity," 1926, from capacity, in reference to electrical conductors, with Latinate agent-noun ending.
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capacitate (v.)
1650s, "make capable; furnish with legal powers," from Latin capacitas (see capacity) + -ate (2). Related: Capacitation.
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*kap- 

Proto-Indo-European root meaning "to grasp."

It forms all or part of: accept; anticipate; anticipation; behave; behoof; behoove; cable; cacciatore; caitiff; capable; capacious; capacity; capias; capiche; capstan; caption; captious; captivate; captive; captor; capture; case (n.2) "receptacle;" catch; catchpoll; cater; chase (n.1) "a hunt;" chase (v.) "to run after, hunt;" chasse; chasseur; conceive; cop (v.) "to seize, catch;" copper (n.2) "policeman;" deceive; emancipate; except; forceps; gaffe; haft; have; hawk (n.); heave; heavy; heft; incapacity; inception; incipient; intercept; intussusception; manciple; municipal; occupy; participation; perceive; precept; prince; purchase; receive; recipe; recover; recuperate; sashay; susceptible.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit kapati "two handfuls;" Greek kaptein "to swallow, gulp down," kope "oar, handle;" Latin capax "able to hold much, broad," capistrum "halter," capere "to grasp, lay hold; be large enough for; comprehend;" Lettish kampiu "seize;" Old Irish cacht "servant-girl," literally "captive;" Welsh caeth "captive, slave;" Gothic haban "have, hold;" Old English hæft "handle," habban "to have, hold."

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liter (n.)
unit of capacity in the metric system, 1797, from French litre (1793), from litron, name of an obsolete French measure of capacity for grain (16c.), from Medieval Latin litra, from Greek litra "pound" (unit of weight), which apparently is from the same Sicilian Italic source as Latin libra (see Libra).
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maneuverability (n.)

also maneuvrability, "capacity for being maneuvered," especially of automobiles and aircraft, 1914, from maneuverable + -ity.

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practicability (n.)

"feasibility, capacity for being practiced," 1720, from practicable + -ity. The earlier word was practicableness (1640s). 

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educability (n.)

"capability of being educated; capacity for receiving instruction," 1821, in phrenology; see educable + -ity.

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paramedical (adj.)

"related to medicine in an auxiliary capacity," 1908, from para- (1) "subsidiary" + medical (adj.).

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