This article extends and expands the existing literature on critical thinking (CT) by both establishing the need for more student-centered research on the topic and reporting on the findings of a CT research project with two central, related goals: 1) To record and analyze undergraduate students' definition of CT and 2) To create an easy-to-use instrument that can assist faculty members across many disciplines identify and understand their students' use of the term so as to improve both teaching and learning. In this article an overview of the limits of existing CT research is followed by a discussion of the project's three, related phases and the initial findings of each. Central to our task is improved classroom teaching. In Phase I an open-ended two-question questionnaire was given to students enrolled in sociology and interdisciplinary courses in two departments during the 2005-2006 academic year (N = 157). In the first question, students were asked simply to "define CT". A qualitative analysis of the data revealed not only wide variation in undergraduate students' use and understanding of the term generally but also wide variation among students in any single classroom. Using the most common student-generated definitions of CT from the survey data, in Phase II we developed and piloted a classroom-ready instrument designed to allow faculty to quickly and simply assess their students' understanding of the term "CT". Faculty who piloted the instrument in Phase III generally found it to be user-friendly and pedagogically valuable. Our overall findings suggest both the need to continue student-centered research in the area of CT and the potential pedagogical value of a student-centered classroom instrument such as the one we created.