In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which Multiplexing|multiplexes a number of Optical Carrier|optical carrier signals onto a single optical fiber by using different wavelengths (i.e. colours) of laser light. This technique enables Duplex (telecommunications)|bidirectional communications over one strand of fiber, as well as multiplication of capacity.
The term wavelength-division multiplexing is commonly applied to an optical carrier (which is typically described by its wavelength), whereas frequency-division multiplexing typically applies to a radio carrier (which is more often described by frequency). Since wavelength and frequency are tied together through a simple directly inverse relationship, the two terms actually describe the same concept.