Defective means having a flaw or fault that prevents something from working the way it should. A defective toy might have a button that doesn't press correctly, or wheels that won't spin. A defective light bulb flickers or burns out immediately instead of glowing steadily.

When something is defective, there's something wrong with it from the start, usually because of a mistake during manufacturing. If you buy a new bicycle and discover the brakes don't work properly, that bicycle is defective. Companies typically replace defective products because they failed to meet their own standards.

The word can describe anything that doesn't function as intended. A detective investigating a crime might discover that a defective lock made it easy for a thief to break in. Scientists studying a disease might find that defective genes cause certain health problems. In each case, defective points to something that should work one way but fails because of an inherent problem.

People also use defect as a noun to describe the actual flaw itself: “The defect in the spacecraft's heat shield caused serious problems during reentry.”