(PSK) A digital modulation scheme that conveys data by changing the phase of a carrier wave. The data can either determine the absolute phase relative to the unmodulated carrier or reference signal ("coherent phase-shift keying", CPSK) or the change in phase ("differential phase-shift keying", DPSK). The number of different phases used determines the amount of data that can be transmitted in each cycle. Each cycle can be considered to constitute one "symbol", e.g. with two possible phases, each cycle carries one bit. The more phases that are used, the less tollerant to noise the transmissions becomes.