With calls for equity and inclusion in STEM fields, diversity is at the forefront of national conversations, particularly in the higher education context. Yet, research has documented the pervasive problem of instructor biases, affecting how they perceive student behavior and position students in the classroom. Using interview data from 30 instructors, we identified five aspects through which they conceptualize diversity as a phenomenon: student characteristics, intelligence mindset, learning environment, instructor actions, and legitimized membership. The variation among these experiences revealed three distinct conceptions: essentialist, functionalist, and existentialist. In the essentialist conception, an instructor perceives student attributes as fixed, utilizes approaches based on equality, and views students as outsiders. In the functionalist conception, an instructor focuses on student features that assist or hinder achievement, finds ways to accommodate student deficits, and considers students as guests that need faculty guidance. In the existentialist conception, an instructor attends to the lived experiences of the students, leverages these experiences as learning opportunities in the classroom, and views students as rightfully present in higher education. Conclusions from this study provide an evidence base to inform professional development programs and bring certain features of diversity into instructors’ awareness to create more inclusive learning environments.