lallation

from The Century Dictionary.

• noun An imperfect pronunciation of the letter r, whereby it is made to sound like l. See lambdacism.

from the GNU version of the Collaborative International Dictionary of English.

• noun An imperfect enunciation of the letter r, in which it sounds like l.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the incorrect pronunciation of the letter "r" so that it sounds like an "l"
• noun baby-talk or gibberish

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun defective articulation of the `l' phoneme or the phoneme `r' is pronounced as `l'
• noun gibberish resembling the sounds of a baby