This study examines middle school mathematics teachers' concept definitions of five geometric objects (prism, pyramid, cone, cylinder, and sphere). The study included 99 middle-grade mathematics teachers in three provinces and districts. The data were gathered through six open-ended questions determined to reveal the participants' Concept Definition (CD) of Geometric Objects (GOs). Via content analysis, codes from the data and categories from the codes were obtained. Results showed that participants could not make the CDs of GOs correctly, generally drew square or rectangular objects, and only included the horizontal and perpendicular states of the GOs (prototype). Participants generally could not establish a relationship between "cylinder and prism" and "cone and pyramid." Some participants saw themselves as inadequate in defining the GOs. Indeed, teachers' explanations and drawings reveal that their professional experiences have been ineffective in defining those concepts.