A slapper is a type of spanking implement akin to a paddle. They are called slappers because of the second layer of leather that adds a "slap" sound on impact.
Slappers are usually longer and thinner than paddles and are made of wood covered with leather. They can be found at many body-safe sex toy stores.
More About Slapper
Because of their smaller size, slappers are easier to aim and control and are appropriate for impact play on the entire body, not just the buttocks.
Slappers provide a sensation similar to those of paddles, but with a little extra "slap" from the second layer of leather. Experienced dominants can provide more sensations by using two slappers at the same time, playing the submissive like a drum.
Depending on the strength of the stroke, a slapper can provide a light sensation that doesn't mark the skin or a hard slap that leaves welts behind. As long as the partner's limits are respected, a slapper can work for both beginning and experienced BDSM partners.