| Abdomen | The front part of the body that lies between the chest and pelvis. |
| Acute | Of short duration and relatively severe. |
| Adjustment | An intervention with the intent of facilitating the body's ability to "right" itself and function more normally |
| Anterior | Toward the front of the body. |
| Arthritis | A general term referring to a condition of the joints. Literally it refers to an inflammation of the joints. There are many types of arthritis. The form which will inflict many people as they age is known as osteoarthritis. Other types include rheumatoid and psoriatic arthritis. |
| Articulation | The connection of bones; a joint. |
| Atlas | The uppermost and most freely movable bone of the spine. |
| Atrophy | A decrease in the size of a normally developed tissue or organ. |
| Autonomic Nervous System | The part of the nerve system that regulates involuntary action, as of the intestines, heart, and glands, and comprises the sympathetic nervous system and the parasympathetic nervous system. |