A bounded stretch of time, and the mark that ends a sentence. Both are ways of saying this much and no further.
Greek περίοδος (periodos, a going around, a cycle), from peri- (around) + hodos (a way, road); a bounded stretch that returns to its own starting point, whether of time or of a sentence.
A bounded stretch of time, or the mark that closes a sentence — two ways of saying this much, and no further.