This term has multiple meanings:
Spontaneous recovery is broadly within the definitions of 'miracle recovery' where there's no apparent cause. In practice, professional physicians only use this term to define a situation in which treatment isn't the obvious cause of the recovery of the patient. The main issue for consumers is that these recoveries are also sometimes attributed to fake 'cures' and other dubious causes, and lack of research tends to make these claims more credible.
Examples of Spontaneous Recovery: