To invalidate something means to make it no longer valid, acceptable, or legally binding. When you invalidate something, you're showing it doesn't count anymore or never should have counted in the first place.

In everyday life, you might see this with tickets or passes. If you lose your library card and get a new one, the library invalidates your old card so nobody else can use it. A sports referee might invalidate a goal if they discover the scoring player was offside. A teacher might invalidate a test score if they catch someone cheating.

The word also describes dismissing someone's feelings or experiences. If your friend tells you they're nervous about an upcoming presentation and someone responds “that's silly, there's nothing to worry about,” they've just invalidated your friend's feelings. They're essentially saying those feelings don't count or shouldn't exist. This use of invalidate helps explain why some conversations feel hurtful: when people invalidate what you're experiencing, they're refusing to acknowledge that your perspective matters.

Scientists use the word when new evidence proves an old theory wrong, invalidating previous conclusions. In all these cases, invalidate means the same thing: taking away something's status, authority, or right to be considered real or true.