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Acceleration: The sudden movement of the brain inside the skull during an impact which causes the tearing of neurons and connections deep inside the brain.

Acceleration/Deceleration: A closed head injury often sustained in car accidents, where the brain smashes forwards and then backwards, rebounding against the walls of the skull, causing damage to both the frontal lobes and the back of the brain.

Acquired Brain Injury: An injury to the brain that probably occurred during birth. An ABI can be caused by direct neurological insult or indirectly via metabolic/systemic illness.

Agnosia: Partial or complete inability to recognize sensory stimuli; perception without meaning.

Alternating Attention: The ability to perform tasks that require rapid switching from one response set to another.

Amnesia: A loss or failure of memory.

Aneurism: Swelling or dilation of an artery due to a weakened wall.

Angular Gyrus: A convolution in the parietal lobe, important language functions and intersensory processing.

Anomia: A difficulty in finding words, especially in naming objects

Anosmia: Loss of sense of smell.

Anosognosia: A diminished self-awareness of problems, resulting from information processing difficulties.

Anoxia: An absence of oxygen supply to an organ's tissues leading to cell death.

Antecedent: A stimulus or event which precedes a behavior.

Anterograde: Inability to remember events subsequent to traumatic brain injury.

Anticipatory Awareness: The ability to anticipate that a problem will occur as a result of having some type of deficit. To anticipate a problem, a person must have both intellectual awareness (i.e. awareness they have a problem) and emergent awareness (i.e. recognition of when problems are actually occurring). The person with problems in this area is unable to realize in advance of their actions (i.e. anticipate) that a given problem will cause a particular problem in the future. Anticipation is one of the important executive functions of the brain.

Apathy: A direct result of brain injury to frontal lobe structures which concern emotion, motivation and forward planning.

Aphasia: Difficulty understanding and/or producing spoken and written language.

Apoptosis: Cell death that occurs naturally as part of normal development, maintenance, and renewal of tissues within an organism.

Apraxia: Impairment in the ability to perform purposeful acts or to manipulate objects without paralysis/paresis; can affect oral, verbal and upper/lower extremity functioning.

Arachnoid Membrane: One of the three membranes that cover the brain; it is between the pia mater and the dura. Collectively, these three membranes form the meninges.

Arachnoid: One of the three membranes holding the brain together within the skull.

Arousal: The ability to stay awake; one part of the attention stage of information processing. An early problem with many survivors is a constant feeling of drowsiness, sleepiness or inability to remain alert. This often improves with time.

Arterial Line: A very thin tube (catheter) inserted into an artery to allow direct measurement of the blood pressure, the amount of oxygen and carbon dioxide in the blood.

Ataxia: A disturbance in the coordination of the muscular movements.

Axon/ Dendrites: Nerve cells in the brain, which look like small hair-like tentacles. The cells communicate with each other by passing electrical and chemical impulses between the tentacles.