The aim of this study was to analyse preservice elementary mathematics teachers' concept definitions of circle, circular region, and sphere concepts in geometrical terms. The research was conducted during the 2016-2017 academic year with 56 preservice mathematics teachers enrolled within a teacher education programme at a state university in Turkey. A test consisting of three open-ended questions regarding circle, circular region, and sphere was used as the study's data collection tool. The preservice teachers' test answers were investigated according to correctness and generalization criteria. Answers regarding each concept were compared with those of the two other concepts. Following the data analysis, semi-structured interviews were conducted with nine of the participant preservice teachers. The findings of the research revealed that the preservice teachers mostly experienced difficulty in defining the sphere. In addition, it was determined that most preservice teachers provided either insufficiently detailed or incorrect answers for the concepts of circle, circular region, and sphere. When the examples of the concepts were examined, it was determined that only a few of the preservice teachers gave appropriate examples of the related concepts.