Willfully means doing something on purpose, especially when you know you probably shouldn't. When someone acts willfully, they're making a deliberate choice, not acting by accident or mistake.

If a student willfully ignores their teacher's instructions, they heard what they were supposed to do but decided not to follow through. If someone willfully breaks a rule, they knew the rule existed and chose to break it anyway. The word carries a sense of stubbornness or defiance: the person insists on their own way despite knowing better.

You might hear willfully in serious contexts, like when someone willfully damages property or willfully disobeys an order. Courts use this word to distinguish between accidents and intentional wrongdoing. If you accidentally knock over a vase while playing catch indoors, that's different from willfully knocking it over because you're angry.