Most analyses of definiteness are based on two important notions: uniqueness and familiarity. Fundamentally, both approaches ascribe some content to the conventional meaning of definite, but not indefinite, descriptions. We explore the effect of determiner choice on listeners’ expectations about possible referents using eye-tracking in the visual world paradigm. We present listeners with temporarily ambiguous definite descriptions where a single referent is unique under the greatest number of possible semantic descriptions. We find that uniqueness is not only a robust notion for describing the meaning of definitess, but also a crucial factor in guiding listeners in the online processing of definite descriptions.