RxPG - Aphasia

Aphasia is a loss or impairment of the ability to produce or comprehend language, due to brain damage. It is usually a result of damage to the language centres of the brain and can be caused by a stroke or physical injury. Depending on the area and extent of the damage, someone may be able to speak but not write, or vice versa, or understand more complex sentences than they can produce.

The brains of young children with brain damage sometimes restructure themselves to use different areas for speech processing, and regain lost function; adult brains are less "plastic" and lack this ability.

Any of the following can be considered aphasia:

inability to comprehend speech
inability to read (alexia)
inability to write (agraphia)
inability to speak, without muscle paralysis
inability to form words
inability to name objects (anomia)
poor enunciation
inappropriate speech, use of jargon or wrong words
inability to repeat a phrase
persistent repetition of phrases
other language impairment

The common types of aphasia are

Broca's aphasia (expressive aphasia)
Wernicke's aphasia (receptive aphasia)
Nominal aphasia (anomic aphasia)
Global aphasia
Conduction aphasia
A few less common varieties include

Transcortical motor aphasia
Subcortical motor aphasia
Transcortical sensory aphasia
Subcortical sensory aphasia
Sources:

National Institute of Health: MEDLINEplus Medical Encyclopdia entry on Speech Impairment (adult)