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

LX - Stands for long wavelength laser.

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