Immersion is a phenomenon experienced by an individual when they are in a state of deep mental involvement in which their cognitive processes (with or without sensory stimulation) cause a shift in their attentional state such that one may experience disassociation from the awareness of the physical world

Definition Source:

Agrawal, S., Simon, A., Bech, S., Bæntsen, K., & Forchhammer, S. (2020). Defining immersion: Literature review and implications for research on audiovisual experiences. Journal of the Audio Engineering Society, 68(6), 404-417.