Proficiency with mathematics requires an understanding of mathematical language. Students are required to make sense of both spoken and written mathematical terms. An essential component of mathematical language involves the understanding of the vocabulary of mathematics in which students connect vocabulary terms to mathematical concepts or procedures. In this brief psychometric report, we developed and tested a measure of mathematics vocabulary for students in the late middle-school grades (i.e., Grades 7 and 8) to determine the reliability of such a measure and to learn how students answer questions about mathematics vocabulary terms. The vocabulary terms on the measure were those terms determined as essential by middle-school teachers for success with middle-school mathematical language. Analysis indicates the measure demonstrated high reliability and validity. Student scores were widely distributed and students, on average, only answered two-thirds of vocabulary terms correctly.