A kill fee compensates a provider for time and effort already invested when a project is cancelled by the client. It's typically a percentage of the total fee — commonly 25-50% depending on how much work was completed.
Kill fees exist because providers turn down other work to commit to your project. Cancellation costs them not just the fee, but the opportunity cost of other bookings they declined.
Professional contracts should specify kill fee terms before work begins. It's an uncomfortable conversation to have, but far less uncomfortable than the dispute that happens without one.