Etymology
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historic (adj.)
1660s, "of or belonging to history," probably a back-formation from historical, perhaps influenced by French historique. Meaning "what is noted or celebrated in history" is from 1794.

Though both historic and historical have been used in both senses by respected authors, now the tendency is to reserve historic for what is noted or celebrated in history; historical for what deals with history. The earliest adjective form of the word in English was historial (late 14c., from Late Latin historialis), which meant "belonging to history; dealing with history; literal, factual, authentic," and also "of historical importance" (early 15c.).
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period (n.)

early 15c., periode, "a course or extent of time; a cycle of recurrence of a disease," from Old French periode (14c.) and directly from Medieval Latin periodus "recurring portion, cycle," from Latin periodus "a complete sentence," also "cycle of the Greek games," from Greek periodos "cycle, circuit, period of time," literally "a going around," from peri "around" (see peri-) + hodos "a going, traveling, journey; a way, path, road," a word of uncertain origin (see Exodus).

Sense of "repeated cycle of events" led to that of "interval of time." From 1712 as "an indefinite part of any continued state or series of events;" by 1727 as "time in which a circuit or revolution (as of a heavenly body) is made." Sense of "episode of menstruation" is by 1829, probably short for period of menstruation (1808), etc.  

The meaning "dot marking end of a sentence" is recorded c. 1600, from the earlier sense of "a complete sentence, from one full stop to another," then "a full pause at the end of a sentence" (1580s). The educational sense of "portion of time set apart for a lesson" is from 1876. The sporting sense "division of a game or contest" is attested by 1898. As an adjective from 1905; period piece is attested from 1911.

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prehistoric (adj.)

also pre-historic, "of or pertaining to times before recorded history, existing in or relating to time antecedent to the beginning of recorded history," 1851, perhaps modeled on French préhistorique; see pre- + historic. Related: Prehistorical.

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ahistoric (adj.)
"not historical, lacking in historical background or justification," 1911, from a- (2) "not" + historic.
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preservationist (n.)
"advocate of protecting existing things," 1905, from preservation + -ist; specifically of historic buildings by 1957.
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historicism (n.)
1856, translating German historismus (by 1835), from historic + -ism. Given various senses 20c. in theology, philosophy, architecture, etc.
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historical (adj.)
early 15c., "of or pertaining to history, conveying information from the past," with -al (1) + Latin historicus "of history, historical," from Greek historikos "historical; of or for inquiry," from historia (see history). For sense differentiation, see historic. Meaning "narrated or mentioned in history" (as opposed to what is fiction or legend) is from 1843. Related: Historically.
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photoperiodism (n.)

"stimulation or inhibition to breeding, etc., based on daily periods of light and darkness," 1920, from photoperiod "period of daily illumination which an organism receives" (1920, from photo- + period) + -ism.

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aeon (n.)
"immeasurable period of time," 1640s; see eon; also see æ (1).
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pentad (n.)

"set of five things considered together," 1650s, from Greek pentas (genitive pentados) "the number five, a group of five," from PIE root *penkwe- "five." Meaning "period of five years" is from 1880; meaning "period of five days" is from 1906, originally in meteorology.

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