The Collaborative International Dictionary of English v.0.48:

Out-patient \Out"-pa`tient\, n.
   1. A patient who is outside a hospital, but receives medical
      aid from it.
      [1913 Webster]

   2. A medical patient who receives treatment at a hospital,
      especially in an emergency room, but is not admitted to
      stay overnight. Distinguished from inpatient, who is
      assigned to a room to reside there while being treated.
      [PJC]

WordNet (r) 3.0 (2006):

outpatient
    n 1: a patient who does not reside in the hospital where he is
         being treated [ant: inmate, inpatient]