- In the transmission of data, a modulation technique in which the phase of the carrier is varied between two or more angles. For instance, to represent binary data only two angles are needed, each 180° apart, and the phase shift could be -90° for a 0, and +90° for a 1. If higher data transfer rates are needed, more phase shifts can be used. For example, four phase angles allow for two bits to be encoded into each signal element, while eight phase angles provide for three bits. When two phase shifts are used, it is called binary phase-shift keying, and when four are used it is called quadrature phase-shift keying. Its abbreviation is PSK.