The Collaborative International Dictionary of English v.0.48:

HMO \HMO\ n. [acronym for Health Maintenance Organization.]
   A health maintenance organization; a form of group health
   insurance that entitles members to services of participating
   hospitals and clinics and physicians. [acronym]

   Syn: health maintenance organization.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

HMO
    n 1: group insurance that entitles members to services of
         participating hospitals and clinics and physicians [syn:
         health maintenance organization, HMO]