An individual wavelength of light for transmitting data on a strand of fiber-optic cable.

Using separate lasers, each tuned to a slightly different frequency, multiple lambdas can be projected down a single fiber strand to carry multiple streams of data.

DWDM (dense wavelength division multiplexing) is the technology for projecting multiple lambda circuits on fiber strands. Currently, 200 lambdas per fiber is common, but thousands are possible.