This study aims to analyze how the Mathematics Teacher Candidates (MTCs) define the "prism" and "pyramid" concepts in geometry. The research was carried out in 2017-2018 academic year with 92 MTCs. This test consists of 3 open-ended questions that MTCs can answer conceptually. The first openended question was as "Define a prism", the second was as "Define a pyramid" and the third was as "Explain the differences between prism and pyramid". The answers of MTCs were investigated according to correctness and generalization criteria. After data analysis, semi-structured interviews were held with 11 MTCs who had ineligible expressions in their definitions. The research revealed that MTCs were unsuccessful in defining prism and pyramid concepts and had difficulties in expressing the differences between these two concepts. Therefore, it was determined that they have insufficient knowledge.