The Collaborative International Dictionary of English v.0.48:

fieldmouse \fieldmouse\, field mouse \field mouse\n.
   1. (Zool.) any nocturnal Old World mouse of the genus
      Apodemus inhabing woods and fields and gardens.
      [WordNet 1.5]

   2. (Zool.) any mouse inhabiting fields, as the campagnol
      and the deer mouse. See Campagnol, and Deer mouse.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fieldmouse
    n 1: any nocturnal Old World mouse of the genus Apodemus
         inhabiting woods and fields and gardens [syn: field
         mouse, fieldmouse]