Teachers perceive student engagement in many ways, and do not necessarily align engagement strategies they employ with their own conceptualization of this construct. This situation is worsened in online learning of which popularity has grown since the emergence of the COVID-19 global pandemic. We explored perceptions of student engagement in online learning, both synchronous and asynchronous, using semi-structured in-depth interviews with K-12 teachers (N=13). We referred to different dimensions (behavioral, emotional, social, and cognitive) and contexts (student, peers, class, school, and home). Findings suggest that student engagement in online learning has unique characteristics: It is mostly behavioral, particularly task-centered; it relates to student-, class-, and home contexts; and teachers take responsibility on- and act upon issues related to it.