1. The way anyone acts or feels in response to something which happens, is said, etc.: When Tim's mother told him what had happened to his sister, his immediate reaction was one of shock and disappointment.

The newspaper received a lot of angry letters in reaction to the editorial about wasting too much money on the elderly.

2. The ability to act and to move quickly in order to avoid some sudden danger: Because Shirley was such a good driver, her quick reactions when another car was coming towards her on the wrong side of the road saved her life and her sister's life.

One of a variety of reactions that can occur as a result of blood transfusions.

The most serious is the response of the recipient when incompatible blood is administered, in which case massive intravascular (occurring within blood vessels) clumping and lysis (destruction) of red blood cells occur.

A response that occurs in the muscles of the body as a result of the stimulation of the nerves in one or more of the internal organs of the body: The delicate post-surgical instruments were able to track the viscerosomatic reactions of the abdominal muscles in the patient's physical structure.