Why is the ninth month called September?
1570s, "equality of rank or status," from Middle French parité (14c.) or directly from Late Latin paritas "equality," from Latin adjective par (genitive paris) "equal" (see pair (n.)). Meaning "condition in which adversaries have equal resources" is from 1955, originally in reference to the U.S.A. and the U.S.S.R.
parity par·i·ty (pār'ĭ-tē)
The state of having given birth to an infant or infants.