To react means to respond to something that happens. When you touch a hot stove, you react by quickly pulling your hand away. When someone tells a funny joke, you might react by laughing. When your teacher announces a surprise quiz, your classmates might react with groans or worried looks.

A reaction is the response itself. In science class, you learn about chemical reactions, where substances combine and change into something new: baking soda and vinegar react to create fizzy bubbles. Your body has reactions too: your immune system reacts to germs by fighting them off, and your eyes react to bright light by squinting.

The word suggests something automatic or immediate rather than carefully planned. You react to a loud noise by jumping, but you respond to an email after thinking about what to write. Some people react to problems by panicking, while others stay calm and think before acting.

A reactor can be a person who reacts easily, or it can be a device where controlled chemical or nuclear reactions take place. Scientists describe someone as reactive if they tend to react strongly or quickly to situations, rather than staying calm and measured.