one who looks after, protects, or defends: the guardian of public morals
(law) someone legally appointed to manage the affairs of a person incapable of acting for himself, as a minor or person of unsound mind
(social welfare) (in England) a local authority, or person accepted by it, named under the Mental Health Act 1983 as having the powers to require a mentally disordered person to live at a specified place, attend for treatment, and be accessible to a doctor or social worker
(often capital) (in England) another word for custos
early 14c., garden; early 15c., gardein, from Anglo-French gardein (late 13c.), from Old French gardien "keeper, custodian," earlier guarden, from Frankish *warding- (see guard (n.)). Guardian angel is from 1630s.