Stand up when the powers that be order you to sit down, and you've given a fine example of defiance. It happens when someone or a group of someones openly flouts or challenges authority.
We owe this lovely descriptive term to the French — specifically to the Old French word defier, which means to defy. (Don't mix it up with deify; that means to make someone or something into a god.) If you've ever studied Latin, you'll spot the fi from fidare "to trust" that inspired the Marines' "semper fi" motto and the popular moniker for pooches––"Fido." Combining fi with de- negates it. So an act of defiance is essentially an act of not trusting.