Children are able to assess the quality of information presented to them, most notably in the domains of causal explanations and arguments. However, children are also presented with another form of verbal information—definitions. Very little empirical work has investigated how children assess and produce definitions. Two experiments explored preschoolers’ comprehension and production of definitions. In Experiment 1, a selective trust paradigm was used to assess 3-year-olds’ (n = 28) and 5-year-olds’ (n = 28) endorsements of informative and uninformative definitions. Participants were provided with two informants: one who always provided a circular definition (e.g., “Silly means when you are silly”) and one who always provided a noncircular definition (e.g., “Silly means when you are goofy”). The 5-year-olds endorsed noncircular definitions over circular definitions for both frequent and infrequent words, but they chose to learn only from informants who provided information about infrequent words. The 3-year-olds, on the other hand, did not systematically endorse either definition type. In Experiment 2, new groups of 3-year-olds (n = 25) and 5-year-olds (n = 24) were asked to provide definitions, and their responses were coded for correctness and circularity. Results demonstrated that 5-year-olds provided more definitions than 3-year-olds. In addition, 5-year-olds provided more noncircular definitions than 3-year-olds for infrequent words but not for frequent words. Together, the results from Experiments 1 and 2 suggest that children’s understanding of definitions emerges during the preschool period. This work presents an important first step in addressing an understudied facet of lexical development.