The Collaborative International Dictionary of English v.0.48:

Celtic \Celt"ic\ (s[e^]lt"[i^]k; k[e^]lt"[i^]k), a. [L.
   Celticus, Gr. Keltiko`s. See Celt.]
   Of or pertaining to the Celts; as, Celtic people, tribes,
   literature, tongue. [Written also Keltic.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Celtic \Celt"ic\, n.
   The language of the Celts.
   [1913 Webster]

   Note: The remains of the old Celtic language are found in the
         Gaelic, the Erse or Irish; the Manx, and the Welsh and
         its cognate dialects Cornish and Bas Breton.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Celtic
    adj 1: relating to or characteristic of the Celts [syn:
           Celtic, Gaelic]
    n 1: a branch of the Indo-European languages that (judging from
         inscriptions and place names) was spread widely over Europe
         in the pre-Christian era [syn: Celtic, Celtic language]