A reaction is the following:

• In physics, a reaction is defined by Newton's third law: "For every action there is an equal and opposite reaction". The idea that any given force has a pair or opposite force.

• Usually, a reaction is any response caused by some other action.
  • Chemical reaction
  • Reflex reaction
  • light reaction
  • Hill reaction
  • Chain reaction

• In politics, the reaction is the attitude of the reactionaries, or their political movement.