To tamper with something means to interfere with it in a harmful or unauthorized way, usually by making changes you shouldn't make. When someone tampers with evidence at a crime scene, they're secretly altering or moving things to change what investigators will discover. If someone tampers with a lock, they're trying to pick it or break it without permission.

The word carries a sense of sneakiness and wrongdoing. You wouldn't say someone tampered with their own bicycle when fixing it, but you would say someone tampered with it if they loosened the brakes as a dangerous prank. A scientist who changes data in an experiment to get better results is tampering with the results. Someone who opens a sealed package and then tries to close it again is tampering with it.

You'll often see warning labels that say “Do not use if seal is broken” or “tamper-evident packaging” on medicine bottles and food containers. These protections exist because tampering can be dangerous.

The word suggests both secrecy and harm: when you tamper with something, you're making unauthorized changes that could cause problems, and you're usually hoping nobody notices what you've done.