An articulation deficit is the inability to produce speech sounds and/or speech sound sequences that are age appropriate.
“Young children often make speech errors. For instance, many young children sound like they are making a ‘w’ sound for an ‘r’ sound (e.g., ‘wabbit’ for ‘rabbit’) or may leave sounds out of words, such as ‘nana’ for ‘banana.’ The child may have an articulation disorder if these errors continue past the expected age.”
Source: http://www.asha.org/public/speech/disorders/speechsounddisorders.htm
Also see “Resources” for age appropriate sounds