A specifications for Gigabit Ethernet over fiber optic cabling (IEEE 802.3z). 1000Base-SX uses short wavelength laser over multimode fiber, as opposed to 1000Base-LX, which uses long wavelength laser over multimode and single-mode fiber. Both have a maximum distance of 550m.

SX - Stands for short wavelength laser.

See also, 1000BaseT, 1000BaseLX  and 1000BaseCX.