Aphasia: One in a group of speech disorders in which there is a defect or loss of the power of expression by speech, writing, or signs, or a defect or loss of the power of comprehension of spoken or written language. See, for example, auditory aphasia.

Less severe forms of aphasia may be called dysphasia. Aphasia, literally, means no speech. From a- meaning lack of + the Greek phasis meaning speech.

See also: Alogia.