Wavelength Division Multiplexing (WDM) - Optical transmission technique in which two or more wavelengths (each carrying its own information), are combined for transmission over a single optical fiber. At the receiving end, the wavelengths are separated and directed to separate receivers.
Wide Area Network - A data network typically extending a LAN outside a building or beyond a campus, over IXC or LEC lines to link to other LANs at remote sites. Typically created by using bridges or routers to connect geographically separated LANs.
Wavelength -

1. The physical distance between two adjacent peaks or valleys in a wave.
2. The property of light that determines its color.