In this study on concepts of geometry, the gestures and mimics used by the mathematics teachers working in school for the deaf to define some mathematical concepts with the sign language were investigated. The research employed the structured interview technique, a qualitative research method. Data were collected through interviews and video recordings with participants. The data were analyzed using the descriptive analysis method in accordance with the parameters set by French linguist Christian Cuxac. Semiotics was utilized to conceptualize and make sense of the ideas behind the signs. As a result of the analysis, signs representing the concepts of "angle, area, diameter, circumference, and perpendicular" were suggested. It was observed that the signs shown in the Turkish Sign Language Dictionary, prepared and distributed to deaf schools by the General Directorate of Special Education and Guidance Services of the Ministry of National Education, are the same. However, the signs for the five concepts show differences. Additionally, during the interviews, teachers stated that many mathematical terms are not included in this dictionary, making it difficult for them to teach effectively. Teachers emphasized that a dictionary containing mathematical terms would be very useful in their courses.