periodyoudaoicibaDictYouDict[period 词源字典]
period: [14] Period means etymologically ‘going round’. It comes via Old French periode and Latin periodus from Greek períodos, a compound noun formed from the prefix perí- ‘round’ and hódos ‘way’ (source also of English episode, exodus [17], and method). The main sense of the word in modern English, ‘interval of time’ (which first emerged in post-classical Latin), comes from the notion of a ‘repeated cycle of events’ (now more obvious in the derivative periodical [17]).
=> episode, exodus, method[period etymology, period origin, 英语词源]
period (n.)youdaoicibaDictYouDict
early 15c., "course or extent of time," from Middle 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, way, journey" (see cede).

Sense of "repeated cycle of events" led to that of "interval of time." Meaning "dot marking end of a sentence" first recorded c. 1600, from similar use in Medieval Latin (in late 16c. English it meant "full pause at the end of a sentence"). Sense of "menstruation" dates from 1822. Educational sense of "portion of time set apart for a lesson" is from 1876. Sporting sense attested from 1898. As an adjective from 1905; period piece attested from 1911.