The aim of this study was to investigate the appropriateness of high school students' definitions. The participants in this study were 269 high school students from a public school in Ordu city, which is on the Black Sea coast of Turkey. The participants were asked to write their definitions with no time constraints. In the analysis of the definitions, students' ability to distinguish necessary and sufficient conditions and their ability to use appropriate mathematical terminology were taken into account. The task used in this study enabled us to mirror students' difficulties and inadequacies about their definitions of a parallelogram. The findings indicated that most of the students defined parallelogram inappropriately because they had used incomplete or incorrect statements. On the other hand, for the appropriate definitions, it was found that the number of uneconomical definitions was almost the same as the number of economical ones. At the end of the study, it was suggested that defining activities should be integrated into curriculums explicitly and should be given importance in our mathematic lessons.