The purpose of this study is to investigate prospective middle school mathematics teachers' (PMTs) formal definitions regarding cylinder and prism and the role of these definitions on their classifications of solids involving (non) prototype examples. Data were collected through questionnaire and interview protocol from three PMTs who were 3rd year students in one of the teacher education programs in Turkey. Data analysis revealed that PMTs mostly used inappropriate terminology in defining cylinder and prism. Despite their inappropriate definitions, one PMT could classify cylinders and non-cylinders among the groups of solids correctly. PMTs who could not classify cylinders correctly made incorrect inclusions regarding the cylinders and prisms.