In Nella Larson's Passing, Irene and Clare are both Black women who are able to pass as white. Irene uses this ability only occasionally, when it suits her to do so. At the beginning of the novel, for instance, she feels faint and goes to a whites-only hotel for a glass of iced tea. Irene lives in Harlem, in a Black community, and is proud of her Black identity. She only passes as white when she is alone. When she is surrounded by other Black people, as she generally is, she is immediately seen by everybody as one of them.
Clare, by contrast, has passed permanently into the white community. Even her husband believes that she is white, and she listens to his racist comments without demur. Clare initially believes that the lifestyle she enjoys as a wealthy white woman is superior to Irene's in every way, and she appears to have rejected her racial roots completely.
The permanence of Clare's passing, at least as far as Irene is concerned, becomes clear when Clare changes her views, considers leaving her racist husband, and wants to rejoin the Black community. Irene believes that this is impossible: Clare is now a white woman and an outsider as far as she and other Black people are concerned.
The novel's title, therefore, plays on the double meaning of the word passing. If someone is able to pass as white, in the sense of convincing others for long enough, she may find that she has passed irrevocably into a different race and a different community.