Effective assessment practice requires clearly defining and operationalizing terminology. We illustrate the importance of this practice by focusing on academic “undermatching”—when students enroll in colleges that are less academically selective than those for which they are academically prepared. Undermatching has been viewed as a potential obstacle in the United States’ goal of increasing degree attainment but operationalizing undermatching is difficult. Using ELS: 2002, a national dataset from the U.S. Department of Education National Center for Education Statistics (NCES, 2014), we developed eight operationalizations of undermatching by altering three commonly used variables. We then compared the number and demographics of students who were identified as undermatched. Differences in operationalizations resulted in significant differences in undermatching by gender, race, parental education, and socioeconomic status. Results of this study illustrate the importance of the need to operationalize terminology used in assessment carefully and consistently.