A serve hit with sidespin that curves the ball wide, pulling the returner off the court. Especially effective from the deuce side where it can swing wide to the forehand or into the body. Less about raw power and more about placement and movement. Federer's slice serve from the ad court was a weapon that could pull returners into the next court over.
"Slice serve swings wide on the deuce side. The returner is practically in the doubles alley trying to reach it."