week is a time unit equal to seven days (Seven-day week), originating in the calendar of the Jews in the 6th century BC and adopted in the Roman, Muslim and various other calendars.

The English word week (:wikt:week) continues an Old English wice, ultimately from a Common Germanic langgem*wik?n-, from a root langgem*wik- "turn, move, change". The Germanic word probably had a wider meaning prior to the adoption of the Roman calendar, perhaps "succession series", as suggested by Gothic (Gothic language) wik? translating taxis "order" in Luke (gospel of Luke) 1:8.

The term "week" is sometimes expanded to refer to other time units comprising a few days. Such "weeks" of between 4 and 10 days have been used historically in various places. Intervals longer than 10 days are not usually termed "weeks" as they are closer in length to the fortnight or the month than to the seven-day week.