The Collaborative International Dictionary of English v.0.48:

Lingual \Lin"gual\ (l[i^][ng]"gwal), a. [L. lingua tongue: cf.
   F. lingual. See Tongue, and cf. Language.]
   1. Of or pertaining to the tongue; uttered by the aid of the
      tongue; glossal; as, the lingual nerves; a lingual letter.
      [1913 Webster]

   2. lying near the tongue; especially, in dentistry, on the
      surface of the tooth next to the tongue. Contrasted with
      buccal, the side of a tooth touching the cheek, i. e.
      the side opposite to the lingual side.
      [PJC]

   Lingual ribbon. (Zool.) See Odontophore.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lingual \Lin"gual\, n.
   A consonant sound formed by the aid of the tongue; -- a term
   especially applied to certain articulations (as those of t,
   d, th, and n) and to the letters denoting them.
   [1913 Webster]

   Note: In Sanskrit grammar certain letters, as [.t], [.t]h,
         [dsdot], [dsdot]h, [.n], are called linguals,
         cerebrals, or cacuminals. They are uttered with the tip
         of the tongue turned up and drawn back into the dome of
         the palate.
         [1913 Webster]

WordNet (r) 3.0 (2006):

lingual
    adj 1: consisting of or related to language; "linguistic
           behavior"; "a linguistic atlas"; "lingual diversity"
           [syn: linguistic, lingual] [ant: nonlinguistic]
    2: pertaining to or resembling or lying near the tongue;
       "lingual inflammation"; "the lingual surface of the teeth"
    n 1: a consonant that is produced with the tongue and other
         speech organs