Disloyalty means breaking faith with someone or something you're supposed to support and stand by. When you're disloyal, you betray trust or abandon a commitment you've made, often when doing so benefits you or makes things easier.

A disloyal friend might share your secrets with others or suddenly side with your critics when you need support. A disloyal teammate might skip practice without explanation or bad-mouth the team behind your back. In historical contexts, disloyalty to one's country meant helping its enemies or betraying military secrets.

Disloyalty is different from simply changing your mind or disagreeing with someone. You can disagree with a friend's decision while still be loyal to them. But if you pretend to support them while secretly working against them, that's disloyalty. It involves deception and betrayal of trust.

The word carries serious weight because loyalty forms the foundation of so many relationships. A single act of disloyalty can destroy years of friendship or trust.