The purpose of this study was to investigate first year elementary mathematics teacher education students' knowledge of prism. For this goal, the participants were asked to define the geometric concept of prism. The participants were 158 first year elementary mathematics teacher education students from a public university in Southern Turkey. The researchers analyzed the participants' definitions of prism. Additionally, 12 of the participants were selected for semi-structured interviews. The data were analyzed via content and frequency analysis techniques. Based on the content analysis, the themes that each participant used were determined. After this evaluation, the frequency of each of the themes was calculated. The findings indicated that the participants experienced difficulties in defining the concept of prism. It was also found out that the participants could not adequately use the mathematical language and define the concept.