Source : Webster's Revised Unabridged Dictionary (1913)

Lallation \Lal*la"tion\, n. [L. lallare to sing lalla, or
   lullaby: cf. F. lallation.]
   An imperfect enunciation of the letter r, in which it sounds
   like l.

Source : WordNet®

lallation
     n 1: defective articulation of the `l' phoneme or the phoneme `r'
          is pronounced as `l'
     2: gibberish resembling the sounds of a baby [syn: {babble}, {babbling}]