The Collaborative International Dictionary of English v.0.48:

Lambdacism \Lamb"da*cism\, n. [L. lambdacismus, Gr. ?, fr.
   la`mbda the letter lambda ([Lambda]).]
   1. A fault in speaking or in composition, which consists in
      too frequent use of the letter l, or in doubling it
      erroneously.
      [1913 Webster]

   2. A defect in pronunciation of the letter l when doubled,
      which consists in giving it a sound as if followed by y,
      similar to that of the letters lli in billion.
      [1913 Webster]

   3. The use of the sound of l for that of r in pronunciation;
      lallation; as, Amelican for American.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lambdacism
    n 1: speech defect involving excessive use or unusual
         pronunciation of the phoneme `l'