Gravidity is defined as the number of times that a woman has been pregnant.
Parity is defined as the number of times that she has given birth to a fetus with a gestational age of 24 weeks or more, regardless of whether the child was born alive or was stillborn.
For example, a woman who is described as 'gravida 2, para 2 (sometimes abbreviated to G2 P2) has had two pregnancies and two deliveries after 24 weeks, and a woman who is described as 'gravida 2, para 0' (G2 P0) has had two pregnancies, neither of which survived to a gestational age of 24 weeks.
How does the medical definition of parity relate to its etymology?
1570s, "equality or equivalence of rank or status," from French parité (14c.) or directly from Late Latin paritas "equality," from Latin adjective par (genitive paris) "equal" (see par (n.)). Meaning "state or condition of being on a level" is from 1610s.