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.
period pe·ri·od (pĭr'ē-əd)
An interval of time characterized by the occurrence of a certain condition, event, or phenomenon.
One of the stages of a disease.
A menstrual period.
A sequence of elements arranged in order of increasing atomic number.
End of story. That is final: Don't ask me again. Period