means Being accepted as a member of a group you don't strictly belong to — whether by race, gender, class, or identity — by presenting in a way that reads as that group.

from From the verb "pass," which traces back through Old French "passer" to Latin "passare," "to step" or "go by" (related to "passus," a pace or step). The sense of slipping through unchallenged is old — like a traveler waved past a checkpoint. By the 19th century in America, "passing" had taken on its loaded social meaning, used especially of people of mixed race who could "pass" as white and so move through a hostile world unobstructed. The word still carries that quiet ache of a gate you weren't meant to walk through, opening anyway.