A coagulant is a substance that causes proteins in a liquid to denature and clump together, turning the liquid into a solid or semi-solid mass. Coagulants can be acids (vinegar, lemon juice, wine), enzymes (rennet), heat itself, or specific salts, and each works by disrupting the protein's normal structure so it unfolds and binds to other proteins nearby.
Coagulation is central to dairy and protein cookery: rennet coagulates milk into curds and whey for cheesemaking, heat coagulates egg proteins into custards and quiches, and coagulants like nigari (magnesium chloride) or lemon juice curdle soy milk into tofu. Agar-agar, a plant-based gelling agent from red algae, is used similarly to set liquids into gels and jellies, especially in Asian cooking.
Coagulation is distinct from emulsification, which stabilizes a mixture of oil and water (as in mayonnaise or hollandaise) without the proteins actually clumping together - the two are sometimes confused because both can thicken a liquid, but they work by different mechanisms.