reaction

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

• noun Any action in resistance or response to the influence of another action or power; reflexive action or operation; an opposed impulse or impression.
• noun In dynamics, a force called into being along with another force, being equal and opposite to it.
• noun 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.
• noun In chem., the mutual or reciprocal action of chemical agents upon each other.
• noun total loss of irritability of the nerve below the lesion; on direct stimulation of the muscle
• noun loss of irritability for very brief currents, such as induction-shocks;
• noun 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);
• noun 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;
• noun a sluggishness of contraction and relaxation.
• noun In pathology, the response of a nerve or muscle to an applied stimulus.
• noun In serumtherapy, the occurrence of an interaction between two substances, as between an agglutinin and an agglutinable substance, or between toxin and antitoxin.
• noun of measuring the rate of certain psychical and psychophysical processes.

from the GNU version of the Collaborative International Dictionary of English.

• noun Any action in resisting other action or force; counter tendency; movement in a contrary direction; reverse action.
• noun (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.
• noun (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.
• noun (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.
• noun (Politics) Backward tendency or movement after revolution, reform, or great progress in any direction.
• noun (Psycophysics) A regular or characteristic response to a stimulation of the nerves.
• noun 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.
• noun (Physiol.) in nerve physiology, the interval between the application of a stimulus to an end organ of sense and the reaction or resulting movement; -- called also physiological time.
• noun (Mech.) a water wheel driven by the reaction of water, usually one in which the water, entering it centrally, escapes at its periphery in a direction opposed to that of its motion by orifices at right angles, or inclined, to its radii.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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