Promoting student understanding of biological concepts is a key part of biology education, and the ability to "understand" a concept forms one of the six categories of the oft-used Bloom's Taxonomy. Despite this, there remains no consensus as to what it means to understand a concept. While several formal definitions have been offered, we investigated how biology instructors and biology education researchers define the term and how they perceived the skill sets needed for a student to understand a concept in the context of assessments. We found that there was no agreement on the definition of understanding, and that responses differed in the cognitive level required to reach "understanding" of a concept. We discuss these findings in the context of Bloom's Taxonomy and variation theory and provide directions for future inquiries. We conclude by discussing implications for biology instructors and the importance of explicitly conveying expectations to better align student and instructor expectations.