A walk-in is a spiritual phenomenon in which the default soul that is born along with a physical body yields to a discarnate entity. By force or by contract or mutually beneficial circumstances, the spirit to literally "walk in" enters the vessel when the 'default' soul bearing the body yields. Because of this exchange, it is commonly referred to as a "walk-in" or use such a term to describe one's being when such is in fact the case. However, there is no solid evidence of how long this has been occurring within history that the author of this explanation knows of at this point in time. It definitely occurred with many individuals as early as the 1960s, with another wave ocurring in the 70s-80s, and more prevalent in the 2000s-onward.
Please feel free to edit and augment with supporting evidence if at all possible.