Passing
Passing is to "pass oneself off" as a member of a group of which one is not a member. Most typically it refers to a strategy adopted by a member of a disenfranchised or discriminated-against group in order to access the privileges of another group, or avoid the discrimination against one's own group. Occasionally a member of a more-privileged group passes as a member of a less-privileged group, which raises very different issues.
The term came to prominence for its use in the United States to refer to light-skinned African-Americans "passing" as white people. The term "passing" is bound up on the historical definitions of "race" in the United States. Socially and culturally, people in the US have been defined as "white" or "non-white", reflecting a social hierarchy in which "white" people are on top, and everyone else is below. Legally, this standard is reflected in the "one-drop rule". While color hierarchies and racism are common in the non-white community, the one-drop rule was a flat barrier, putting people on one or the other side of a system of legal and political discriminations, as well as legally-supported social, cultural, and economic discriminations. In this environment, there was real incentive for light-skinned African-Americans to pass.
Despite the significant incentives for individuals to pass, they were also subject to significant criticism from within their original community, a line of criticism that effectively inscribed passing people as race traitors. (This original term should be distinguished from the modern anti-racist movement.) People who passed left their originating community, sometimes abandoning it and their families entirely; passing was therefore seen in opposition to movements to lift up which encouraged individuals who achieved success to give back to their communities by helping the community or other individuals. While individuals who passed could be reviewed sympathetically, many criticized or decried the phenomenon.
The term "passing" has also been widely adopted to describe women passing as men. While these women have been excoriated as unfeminine or unnatural, the "traitor" criticism levied against African-Americans has not been a major thread of analysis of passing women (or passing men, for that matter).
In queer communities, the phenomena of "passing" has a unique nuance. While discriminated against, the queer community also suffers from invisibility (see closeted and outing). Moreover, for many (or most) LGBT people, their sexuality is not immediately apparent and visible, and passing is a default state. Overt gender-nonconformity (e.g., butch women), or wearing LGBT-identified insignia (rainbows, pink triangles, etc.), is thus adopted to prevent passing, and is a sign of community and identity pride.