Avian vocal mimicry is a conspicuous and spectacular form of animal communication, occurring in many distantly related birds around the globe. However, the proximate and ultimate causes of vocal mimicry are poorly understood. We argue that progress has been impeded by conceptual confusion over what constitutes vocal mimicry. Therefore, we propose a modified version of Vane-Wright’s widely used definition of mimicry (1980), which defines mimicry from the perspective of the selective agent of mimicry – the receiver. We suggest that our new definition (Dalziell et al., 2014) provides a functionally coherent framework for determining what constitutes vocal mimicry, while taking account of the diversity of avian vocalisations that incorporate heterospecific sounds. Importantly, this new definition is not restricted to vocal mimicry in birds, and thus it can help unify the diversity of mimetic signals across sensory modalities and taxonomic groups.