n. [ Pref. ab- + reaction, after G. Abreagirung. ] (Psychotherapy) the purging of emotional tensions. See Catharsis, below.
Syn. -- catharsis, katharsis [ Webster 1913 Suppl. WordNet 1.5 ]

n. an apparatus in which a suspension of microorganisms in a liquid are used to perform chemical reactions, as in synthesis of pharmaceutical agents or the conversion of harmful waste to less harmful substances. The reactor consists of a vessel to contain the suspension of microorganisms, plus a variety of attached devices used to control the reaction. [ PJC ]

. an immediate unthinking emotional reaction produced by an event or statement to which the reacting person is highly sensitive; -- in persons with strong feelings on a topic, it may be very predictable. [ PJC ]

n. A nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy. Syn. -- fusion, nuclear fusion. [ WordNet 1.5 ]

n. A process that alters the energy or structure or composition of an atomic nucleus. [ WordNet 1.5 ]

n. (phys.) Any of several devices that maintain and control a sustained nuclear fission chain reaction, for the production of energy, heat, or artificial elements, or for research purposes. The main fuel sustaining the reaction and consumed by the process is typically uranium or plutonium. Syn. -- reactor, atomic pile, fission reactor, nuclear pile [ WordNet 1.5 +PJC ]

v. t. To act beforehand; to perform previously. [ 1913 Webster ]

n. Previous action. [ 1913 Webster ]

v. t. To act or perform a second time; to do over again; to reenact; as, to react a play; the same scenes were reacted at Rome. [ 1913 Webster ]

v. i. 1. To return an impulse or impression; to resist the action of another body by an opposite force; as, every body reacts on the body that impels it from its natural state. [ 1913 Webster ]

2. To act upon each other; to exercise a reciprocal or a reverse effect, as two or more chemical agents; to act in opposition. [ 1913 Webster ]

n. [ React + -ance. ] (Elec.) The influence of a coil of wire upon an alternating current passing through it, tending to choke or diminish the current, or the similar influence of a condenser; inductive resistance. Reactance is measured in ohms. The reactance of a circuit is equal to the component of the impressed electro-motive force at right angles to the current divided by the current, that is, the component of the impedance due to the self-inductance or capacity of the circuit. [ Webster 1913 Suppl. ]

(Elec.) A choking coil. [ Webster 1913 Suppl. ]

n. [ Cf. F. réaction. ] 1. Any action in resisting other action or force; counter tendency; movement in a contrary direction; reverse action. [ 1913 Webster ]

2. (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. [ 1913 Webster ]

3. (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. [ 1913 Webster ]

4. (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. [ 1913 Webster ]

Reaction is always equal and opposite to action, that is to say, the actions of two bodies upon each other are always equal and in opposite directions. Sir I. Newton (3d Law of Motion). [ 1913 Webster ]

5. (Politics) Backward tendency or movement after revolution, reform, or great progress in any direction. [ 1913 Webster ]

The new king had, at the very moment at which his fame and fortune reached the highest point, predicted the coming reaction. Macaulay. [ 1913 Webster ]

6. (Psycophysics) A regular or characteristic response to a stimulation of the nerves. [ Webster 1913 Suppl. ]

7. 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. [ PJC ]

Reaction time (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. --
Reaction wheel (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. [ 1913 Webster ]

a. Being, causing, or favoring reaction; as, reactionary movements. [ 1913 Webster ]

n.; pl. Reactionaries One who favors reaction, or seeks to undo political progress or revolution. [ 1913 Webster ]

n. A reactionary. C. Kingsley. [ 1913 Webster ]

a. [ Cf. F. réactif. ] Having power to react; tending to reaction; of the nature of reaction. -- Re*act"ive*ly, adv. -- Re*act"ive*ness, n. [ 1913 Webster ]

n. 1. (Elec.) A choking coil. [ Webster 1913 Suppl. ]

2. A nuclear reactor. [ PJC ]

{ or or } [ After Fernand Widal (b. 1862), French physician. ] (Med.) A test for typhoid fever based on the fact that blood serum of one affected, in a bouillon culture of typhoid bacilli, causes the bacilli to agglutinate and lose their motility. [ Webster 1913 Suppl. ]