epicenter n.

before vowels reduced to ep-, before aspirated vowels eph-, word-forming element meaning "on, upon, above," also "in addition to; toward, among," from Greek epi "upon, at, close upon (in space or time), on the occasion of, in addition," also "after," from PIE *epi, *opi "near, at, against" (source also of Sanskrit api "also, besides;" Avestan aipi "also, to, toward;" Armenian ev "also, and;" Latin ob "toward, against, in the way of;" Oscan op, Greek opi- "behind;" Hittite appizzis "younger;" Lithuanian ap- "about, near;" Old Church Slavonic ob "on"). A productive prefix in Greek; also used in modern scientific compounds (such as epicenter).

late 14c., "middle point of a circle; point round which something revolves," from Old French centre (14c.), from Latin centrum "center," originally the fixed point of the two points of a drafting compass (hence "the center of a circle"), from Greek kentron "sharp point, goad, sting of a wasp," from kentein "stitch," from PIE root *kent- "to prick" (source also of Breton kentr "a spur," Welsh cethr "nail," Old High German hantag "sharp, pointed").

The spelling with -re was popularized in Britain by Johnson's dictionary (following Bailey's), though -er is older and was used by Shakespeare, Milton, and Pope. The meaning "the middle of anything" attested from 1590s. Figuratively, "point of concentration" (of power, etc.), from 1680s. The political use, originally in reference to France, "representatives of moderate views" (between left and right) is from 1837. Center of gravity is recorded from 1650s. Center of attention is from 1868.

chiefly British English spelling of epicenter; for spelling, see -re.