Academic writing is often referred to as “formal,” but the teaching and assessment of formality can be challenging as formality has been conceptualized in many ways. The goal of this study is to explore the elusive construct of formality in the context of academic writing, especially with regard to what formality means to academic writing instructors. We used instructors’ perceptions of formality (i) to identify relationships between the use of linguistic features in academic texts and perceptions of formality and (ii) to determine the extent to which the situational characteristics of texts (e.g., differences in audience, purpose, and discipline) are related to perceptions of formality. Specifically, we asked 72 academic writing instructors to rate the formality level of 60 short academic text excerpts on a five-point scale. The excerpts were sampled from two publication types (university textbooks, journal articles) in three disciplines (psychology, biology, history). Overall, the results indicate that perceptions of formality can be explained by both linguistic features and situational characteristics. As linguistic features and situational characteristics are intertwined, differences in perceptions of formality seem to be functionally motivated. Implications for the teaching of academic writing are discussed.