Residual describes what remains after most of something is gone. It’s an almost formal word for what’s leftover. If you’ve gotten over your breakup but you still have the urge to kick your ex, then you have some residual bitterness.
Residual comes from the Old French root word residu, which means "a remainder." That's why residual can be something that is left behind, like the residual grime on your kitchen counter from wiping it with a dirty sponge or residual feelings of guilt that remain after you dumped your girlfriend because you were rude. A residual is also a payment to a performer, writer, or director of a television show every time the commercial airs; it’s a kind of royalty.