A term mentioned a lot in American television talk shows and by pop 'psychologists' like Dr Phil. A psuedo metaphysical cliché based on normative assumptions about personal relationships.
It is an elusive and subjective term and any definition would be to apply a generalisation to personal relationships. It could be argued that searching for a 'soulmate' will only lead to frustration because it is questionable that such an entity even exists.
Mary has become depressed because she believes her past relationships failed, as she was unable to locate her supposed soulmate out of the billions of inhabitants of planet Earth.
As a well known song states "It's got to be, perfect and I won't take anything less". Mary follows this statement too closely and is lead to constant disappointment, because she believes life is like a romance novel.