Colloquially, a player who hits the ball solidly is said to have gotten the "sweet spot" of the bat on the ball. The sweet spot classification quantifies that as a batted-ball event with a launch angle between eight and 32 degrees.
A player's sweet spot percentage -- or how often he produces a batted-ball event with a launch angle between eight and 32 degrees -- is presented on Statcast leaderboards under SwSp%.
Why it's useful
While the sweet spot classification does not include exit velocity and thus doesn't tell the complete story of a batted-ball event, players with a high sweet spot percentage are putting themselves in greater position to succeed. In 2018, Major Leaguers posted a 1.099 slugging percentage on batted balls with a launch angle between eight and 32 degrees.
Sweet spot percentage can be used in concert with hard-hit rate -- the percentage of a player's batted balls that have an exit velocity of 95 mph or higher. For a batted-ball classification that takes into account both launch angle and exit velocity, check out barrels.