The Collaborative International Dictionary of English v.0.48:

Intercostal \In`ter*cos"tal\, a. (Anat. & Physiol.)
   Between the ribs; pertaining to, or produced by, the parts
   between the ribs; as, intercostal respiration, in which the
   chest is alternately enlarged and contracted by the
   intercostal muscles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

intercostal
    adj 1: located or occurring between the ribs; "intercostal
           muscles"
    n 1: muscles between the ribs; they contract during inspiration
         [syn: intercostal, intercostal muscle, musculus
         intercostalis]