zhōu
cycle
Original meaning:
to carve jade
Depicts a carved jade object, now written as 彫/琱. 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.
Components
口 Distinguishing component
kǒu
mouth
Evolution

Oracle script
(~1250-1000 BC)
Bronze script
Early Western Zhou (~1000 BC)
Seal script
Shuowen (~100 AD)
Clerical script
Eastern Han dynasty (25-220 AD)Regular script
ModernDefinitions
Sources
季旭昇《說文新證》p.99-100
林志強《《文源》評注》p.134