A reaction is usually any response caused by some other action:

• Chain
• Chemical
• Emotional
• Hill
• Light
• Nuclear
• Physical reaction - 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.
• Reflexive
• Political - The reaction is the attitude of the reactionaries, or their political movement
• Risk management - The reaction is a response after the hazard. Prevention is a response before the hazard materialization.
• see also Adverse drug reaction

Reaktion | Reaktion | Reaktsioon | Réaction | Reakcija | 反応 | Reakcja | Reação | 反應