How tightly the strings are strung in a racquet, measured in pounds or kilograms. Higher tension gives more control but less power. Lower tension gives more power and spin but less precision. Most pros string between 50-60 lbs. Players adjust tension based on court surface, weather, and how they're feeling. Fresh strings versus dead strings is a real and noticeable difference.
"He's strung his racquet at 55 pounds for the clay. A few pounds looser than his hard court setup for more spin."