A victory won at excessive cost.
Example:
The coach regarded their win as a Pyrrhic victory, as his best players sustained injuries that would sideline them for weeks.
Pyrrhic victories take their name from Pyrrhus, the king of Epirus, an ancient country in northwest Greece. Pyrrhus defeated the Romans at the Battle of Ausculum (279 B.C.) but lost all of his best officers and many men. He is said to have exclaimed after the battle "One more such victory and we are lost."