zhōu
cycle
Original meaning:
farmland
Originally depicted farmland. The meaning later shifted to "circumference" and "cycle". Later the 口 component was added when 周 was used to refer to a geographical region to indicate that the character was only "mouthed", i.e. used for its sound. In simplified Chinese 周 is also used to mean "week", while in traditional Chinese this meaning is written with a separate character 週.
口 Distinguishing component
kǒu
mouth
Oracle script
Bronze script
Seal script
Clerical script
Regular script
季旭昇《說文新證》p.99-100