Curiosity and interest are at the core of human inquiry. However, controversies remain about how best to conceptualize these constructs. I propose to derive definitions by attending to the common core of typical usages of the two terms. Using this approach, curiosity can be defined as a psychological state that includes three components: recognition of an information gap, anticipation that it may be possible to close it, and an intrinsically motivated desire to do so. Interest can be more broadly defined as intrinsically motivated engagement with any specific object, content, or activity. The two definitions imply that curiosity is a special case of interest. Furthermore, I propose to use the state-trait distinction to distinguish between momentary and enduring forms of both curiosity and interest, which makes it possible to treat state versus trait curiosity and interest in conceptually parallel ways. To make further progress in understanding the two constructs, research is needed that investigates their affective dynamics and their generalizability across age-related and socio-cultural contexts.