Research has demonstrated ambiguity about the definition of hooking up among college students. The current research examined whether there were multiple definitions of hooking up among college students and how different definitions might be associated with the participant's own hooking up behavior and normative perceptions of peer hooking up behavior. A random sample (N = 1,468) of undergraduates (56.4% female) completed a Web-based survey composed of measures of drinking and sexual behavior. Open-ended definitions of hooking up were content-coded and analyzed using a mixture model to explore discrete definitions of hooking up among college students. Findings indicated three clusters of student definitions of hooking up: Cluster 1 had the broadest definition, referring to sex in general, not specific sexual acts, and to making out. Cluster 2 placed an emphasis on interpersonal and social aspects. Cluster 3 defined hooking up as sex with notable references to specific sexual acts. Results further indicated that hooking up behavior and normative perceptions differentiated these three groups of definitions. Clinical implications regarding the inconsistency of student definitions of hooking up and how they may impact negative consequences associated with hooking up are discussed.