Why is the ninth month called September?
late 14c., from Latin circumferentia, neuter plural of circumferens, present participle of circumferre "to lead around, take around, carry around," from circum "around" (see circum-) + ferre "to carry" (see infer). A loan-translation of Greek periphereia "periphery, the line round a circular body," literally "a carrying round" (see periphery). Related: Circumferential.
The measure of the distance around a circle.