Limited-slip differential. A differential that distributes power between the left and right wheels to reduce wheel spin under acceleration.
LSD (limited-slip differential) is a differential designed to distribute power between left and right wheels more aggressively than an open differential. An open differential allows one wheel to spin freely if it loses traction, which kills acceleration. An LSD locks (or partially locks) the two wheels together when slip is detected, transferring more torque to the wheel with grip.
LSD types include clutch-pack (mechanical, common in performance cars and trucks), Torsen (mechanical, gear-based, smooth operation), helical (similar to Torsen, common in modern cars), viscous (silicone-fluid based, simpler but less aggressive), and electronic (computer-controlled clutch packs, increasingly common in modern AWD systems). Each has different characteristics suited to different applications.
Most performance cars come with an LSD as standard equipment. Honda Civic Type R, Subaru WRX STI, Nissan Z, and most BMW M cars all have LSDs. Aftermarket LSDs are common upgrades on cars without one (Mazda Miata, older RWD cars), and LSD upgrades for cars that already have factory LSDs are popular in racing applications.