A walk-in is a new-age concept of a person whose consciousness has either been replaced or merged with a new consciousness, either temporarily or permanently.
The Prism of Lyra defines a walk-in as follows: "Generally, a walk-in experience will take two forms. The most common can be considered a soul braid in which more of an individual's own energy (higher self or future aspects) is brought in and integrated with the personality. The second and more rare is an instance in which an actual 'separate' consciousness inhabits a human body while the original soul departs the physical plane. This is a highly personal experience where the newly arrived entity does not need to proclaim its origin or status. Quite frequently the soul-braid experience is mistaken for the walk-in exchange because the personality construct (as biochemically encoded within the brain) experiences much the same manifestation in either case." See also: soul braid.