Launch rate, also known as Damage % in Super Smash Bros. and Damage Ratio in Super Smash Bros. Melee and Super Smash Bros. Brawl, is a feature in all Super Smash Bros. games that alters how much knockback is given and received (despite the name in Melee and Brawl, damage is unaffected). All knockback in a match is multiplied by the launch rate value, which by default is 1.0 but can be set from as low as 0.5 (attacks have half knockback) to as high as 2.0 (attacks have double knockback). The minimal launch rate is ideal for combos while the maximum launch rate is ideal for OHKOs. In Smash 4, attacks that deal set knockback are unaffected by changes to the launch rate, allowing those attacks to still combo into each other.
Launch rate is not to be confused with handicap, which is another feature that increases/decreases knockback, but in an entirely different way.
There has been some discussion among Brawl's competitive community as to whether using a launch rate value other than the default 1.0 results in a more healthy metagame; 1.1 is a commonly-mentioned value claimed to make many polarizing attack chains (such as using Sheik's forward tilt into itself endlessly) less viable while also shortening the length of stocks. The idea has never attained serious consideration among tournament organizers.