The term for a substance that inhibits a chemical reaction is called an inhibitor. Inhibitors can slow down or prevent reactions by affecting the activity of enzymes or other chemical agents involved in processes. There are various types of inhibitors, such as competitive inhibitors that compete with substrates for enzyme binding sites, and non-competitive inhibitors that bind to enzymes at other sites, altering their function. Inhibitors are crucial in pharmaceuticals, as they can regulate biological pathways and treat numerous diseases.