The term interaction is field-defining, yet surprisingly confused. This essay discusses what interaction is. We first argue that only few attempts to directly define interaction exist. Nevertheless, we extract from the literature distinct and highly developed concepts, for instance viewing interaction as dialogue, transmission, optimal behavior, embodiment, and tool use. Importantly, these concepts are associated with different scopes and ways of construing the causal relationships between the human and the computer. This affects their ability to inform empirical studies and design. Based on this discussion, we list desiderata for future work on interaction, emphasizing the need to improve scope and specificity, to better account for the effects and agency that computers have in interaction, and to generate strong propositions about interaction.