Reaction wheel

Reaction Re*ac"tion (r[-e]*[a^]k"sh[u^]n), n. [Cf. F. r['e]action.] 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]