The use of the term 'immersion' to describe a multitude of varying experiences in the absence of a definitional consensus has obfuscated and diluted the term. This paper presents a non-exhaustive review of previous work on immersion on the basis of which a definition of immersion is proposed: a state of deep mental involvement in which the subject may experience disassociation from the awareness of the physical world due to a shift in their attentional state. This definition is used to contrast and differentiate interchangeably used terms such as presence and envelopment from immersion. Additionally, an overview of prevailing measurement techniques, implications for research on immersive audiovisual experiences, and avenues for future work are discussed briefly.