reaction

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A response to a stimulus.
2. n. The state resulting from such a response.
3. n. A reverse or opposing action.
4. n. A tendency to revert to a former state.
5. n. Opposition to progress or liberalism; extreme conservatism.
6. n. Chemistry A change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.
7. n. Physics A nuclear reaction.
8. n. Physics An equal and opposite force exerted by a body against a force acting upon it.
9. n. The response of cells or tissues to an antigen, as in a test for immunization.

Century Dictionary and Cyclopedia

1. n. Any action in resistance or response to the influence of another action or power; reflexive action or operation; an opposed impulse or impression.
2. n. In dynamics, a force called into being along with another force, being equal and opposite to it. All forces exist in pairs; and it is a fundamental law (Newton's third law of motion) in mechanics that “action and reaction are always equal and contrary,” or that the mutual actions of two bodies are always equal and exerted in opposite directions. This law was announced, in the form that the quantity of motion is preserved in all percussion, simultaneously in 1669 by Christian Huygens, John Wallis, and Sir Christopher Wren, but was experimentally proved by Wallis only.
3. n. Action contrary to a previous influence, generally greater than the first effect; in politics, a tendency to revert from a more to a less advanced policy, or the contrary.
4. n. In chem., the mutual or reciprocal action of chemical agents upon each other.
5. n. total loss of irritability of the nerve below the lesion; on direct stimulation of the muscle
6. n. loss of irritability for very brief currents, such as induction-shocks;
7. n. retention and even increase of irritability for making and breaking of currents of longer duration (this galvanic irritability also becomes lost in the terminal stages of the severest forms);
8. n. increase of irritability for making currents at the anode as compared with the cathode, so that the anode closing contraction may exceed the cathode closing contraction;
9. n. a sluggishness of contraction and relaxation.
10. n. In pathology, the response of a nerve or muscle to an applied stimulus.
11. n. In serumtherapy, the occurrence of an interaction between two substances, as between an agglutinin and an agglutinable substance, or between toxin and antitoxin.
12. n. of measuring the rate of certain psychical and psychophysical processes.

Wiktionary

1. n. An action or statement in response to a stimulus or other event
2. n. chemistry A transformation in which one or more substances is converted into another by combination or decomposition

GNU Webster's 1913

1. n. Any action in resisting other action or force; counter tendency; movement in a contrary direction; reverse action.
2. n. (Chem.) The mutual or reciprocal action of chemical agents upon each other, or the action upon such chemical agents of some form of energy, as heat, light, or electricity, resulting in a chemical change in one or more of these agents, with the production of new compounds or the manifestation of distinctive characters. See Blowpipe reaction, Flame reaction, under Blowpipe, and Flame.
3. n. (Med.) An action induced by vital resistance to some other action; depression or exhaustion of vital force consequent on overexertion or overstimulation; heightened activity and overaction succeeding depression or shock.
4. n. (Mech.) The force which a body subjected to the action of a force from another body exerts upon the latter body in the opposite direction.
5. n. (Politics) Backward tendency or movement after revolution, reform, or great progress in any direction.
6. n. (Psycophysics) A regular or characteristic response to a stimulation of the nerves.
7. n. An action by a person or people in response to an event. The reaction may be primarily mental (“ a reaction of surprise”) but is usually manifested by some activity.

WordNet 3.0

1. n. doing something in opposition to another way of doing it that you don't like
2. n. (mechanics) the equal and opposite force that is produced when any force is applied to a body
3. n. extreme conservatism in political or social matters
4. n. a bodily process occurring due to the effect of some antecedent stimulus or agent
5. n. an idea evoked by some experience
6. n. (chemistry) a process in which one or more substances are changed into others
7. n. a response that reveals a person's feelings or attitude