In recent years, science education researchers have increasingly studied the ways in which students "make sense" of science. However, although researchers might all agree intuitively on what it looks like, the literature on sensemaking is theoretically fragmented. In this paper, we address this fragmentation by proposing a coherent definition, arguing that sensemaking is the process of building an explanation to resolve a perceived gap or conflict in knowledge. We then present an overview of three primary approaches to describing sensemaking in the science education research literature, arguing that this body of literature has conceptualized sensemaking as a stance toward science learning, a cognitive process, and a particular form of discourse, and showing how the definition incorporates each conceptualization. We conclude by describing how sensemaking is distinct from general categories of activities like "thinking," "learning," and several scientific practices. We also highlight the implications of this definition for science instruction and future sensemaking research.