Whether students view intelligence as a fixed or malleable trait (i.e., their "mindset") has significant implications for their responses to failure and academic outcomes. Despite a long history of research on mindset and its growing popularity, recent meta-analyses suggest that mindset does a poor job of predicting academic outcomes for undergraduate populations. Here, we present evidence that these mixed results could be due to ambiguous language on the mindset scale. Specifically, the term "intelligence" is a referent in every item of the mindset scale but is never defined, which could result in differing interpretations and measurement error. Therefore, we conducted an exploratory, qualitative study to characterize how undergraduate students define intelligence and how their definitions may influence how they respond to the mindset scale. We uncovered two distinct ways that undergraduates define intelligence: knowledge and abilities (e.g., ability to learn, solve problems). Additionally, we found that students' definitions of intelligence can vary across contexts. Finally, we present evidence that students who define intelligence differently also interpret and respond to the items on the mindset scale differently. We discuss implications of these results for the use and interpretation of the mindset scale with undergraduate students.