The Collaborative International Dictionary of English v.0.48:

Interlineal \In`ter*lin"e*al\, Interlinear \In`ter*lin"e*ar\, a.
   [Cf. LL. interlinearis, F. interlin['e]aire.]
   Contained between lines; written or inserted between lines
   already written or printed; containing interlineations; as,
   an interlinear manuscript, translation, etc. --
   In`ter*lin"e*ar*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interlinear
    adj 1: written between lines of text [syn: interlinear,
           interlineal]