Wavelength Division Multiplexing (WDM) is a technology that allows multiple signals to be transmitted simultaneously over a single optical fiber. It achieves this by using different wavelengths, or colors, of light to carry separate data streams. This increases the capacity of the fiber, enabling more information to be sent without needing additional physical cables. WDM is commonly used in telecommunications to enhance the efficiency of data transmission over long distances. By combining various wavelengths at the transmitter and separating them at the receiver, WDM systems can significantly improve bandwidth and reduce costs associated with infrastructure expansion.