A retardant is a substance that slows down or delays something, usually a chemical reaction or process.

The most familiar example is fire retardant, a chemical that slows the spread of flames. Firefighters drop fire retardant from airplanes onto wildfires: it looks like pink or red slime coating the trees and ground, creating a barrier that helps stop the fire from advancing. Buildings often use fire-retardant materials in walls and furniture to give people more time to escape if a fire starts. The retardant doesn't make things fireproof, but it buys precious time by making materials harder to ignite and slower to burn.

Flame retardants appear in many everyday items, from pajamas to couch cushions to electronics. Scientists add these chemicals during manufacturing to meet safety standards. Some retardants work by releasing water vapor when heated, which cools the fire. Others form a protective coating that blocks oxygen from feeding the flames.

The word retardant can also be an adjective, describing anything designed to delay a process: rust-retardant coatings slow corrosion on metal, and mold-retardant treatments slow fungus growth. Whatever the type, a retardant's job is always the same: to slow things down when speed could mean disaster.