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.).
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.
"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.
