<unk> s or speech impediments, as they are also called, are a type of communication disorders where 'normal' speech is disrupted. This can mean stuttering, lisps, etc.  Someone who is totally unable to speak due to a<unk>  is considered mute.
Speech disorder
Speech sound disorder
Language disorder
Hearing impairment
Language delay
Nonverbal learning disorder
Mental retardation
Auditory processing disorder