The excessive "jargon" load in biology may be a hurdle for developing conceptual understanding as well as achieving core competencies such as scientific literacy and communication. Little work has been done to characterize student understanding of biology-specific jargon. To address this issue, we aimed to determine the types of biology jargon terms that students struggle with most, the alignment between students' perceived understanding and performance defining the terms, and common errors in student-provided definitions. Students in two biology classes were asked to report their understanding of, and provide definitions for, course-specific vocabulary terms: 1276 student responses to 72 terms were analyzed. Generally, students showed an overestimation of their own understanding. The least accurate self-assessment occurred for terms to which students had substantial prior exposure and terms with discordant meanings in biology versus everyday language. Students were more accurate when assessing their understanding of terms describing abstract molecular structures, and these were often perceived as more difficult than other types of terms. This research provides insights about which types of technical vocabulary may create a barrier to developing deeper conceptual understanding, and highlights a need to consider student understanding of different types of jargon in supporting learning and scientific literacy.