What is Aphasia?

Legal Definition
Aphasia is an inability to comprehend and formulate language because of damage to specific brain regions. This damage is typically caused by a cerebral vascular accident (stroke), or head trauma, however these are not the only possible causes. To be diagnosed with aphasia, a person's speech or language must be significantly impaired in one (or several) of the four communication modalities following acquired brain injury or have significant decline over a short time period (progressive aphasia). The four communication modalities are auditory comprehension, verbal expression, reading and writing, and functional communication.

The difficulties of people with aphasia can range from occasional trouble finding words to losing the ability to speak, read, or write; intelligence, however, is unaffected. Expressive language and receptive language can both be affected as well. Aphasia also affects visual language such as sign language. In contrast, the use of formulaic expressions in everyday communication is often preserved. One prevalent deficit in the aphasias is anomia, which is a deficit in word finding ability.

The term "aphasia" implies that one or more communication modalities have been damaged and are therefore functioning incorrectly. Aphasia does not refer to damage to the brain that results in motor or sensory deficits, as it is not related to speech (which is the verbal aspect of communicating) but rather the individual's language. An individual's "language" is the socially shared set of rules as well as the thought processes that go behind verbalized speech. It is not a result of a more peripheral motor or sensory difficulty, such as paralysis affecting the speech muscles or a general hearing impairment.

Aphasia is from Greek a- ("without") + phásis (φάσις, "speech"). The word aphasia comes from the word ἀφασία aphasia, in Ancient Greek, which means "speechlessness", derived from ἄφατος aphatos, "speechless" from ἀ- a-, "not, un" and φημί phemi, "I speak".
-- Wikipedia
Legal Definition
In medical jurisprudence. Loss of the faculty or power of articulate speech; a condition in which the patient, while retaining intelligence and understanding and with the organs of speech unimpaired, is unable to utter articulate words or unable to vocalize the particular word which is in his mind and which he wishes to use or utters words different from those he believes himself to be speaking or (in "sensory aphasia") is unable to understand spoken or written language. The seat of the disease is in the brain, but It is not a form of insanity.
-- Black's Law Dictionary
Legal Definition
A mental affliction whereby one loses comprehension of the sense of words and of familiar objects.
-- Ballentine's Law Dictionary