We examined the relationship between trait ambiguity and self-peer agreement in personality judgment. In Study 1, self-peer agreement was lower on ambiguous traits (those with many behavioral referents) than on unambiguous ones (those with few behavioral referents). This finding was partially moderated by the level of friendship between peers. These results suggest that people disagree in their judgments because they use idiosyncratic trait definitions when making judgments on ambiguous traits. Study 2 tested this explanation by exploring self-peer agreement when participant pairs were forced to use the same trait definition versus different ones when judging themselves and each other. Forcing participants to use the same trait definition increased the degree to which their judgments covaried with one another. Discussion centers on the cognitive and motivational forces that can influence the degree to which personality judgments differ.