In telecommunication, the term phase-shift keying (PSK) has the following meanings:

1. In digital transmission, angle modulation in which the phase of the carrier is discretely varied in relation either to a reference phase or to the phase of the immediately preceding signal element, in accordance with data being transmitted.

2. In a communications system, the representation of characters, such as bits or quaternary digits, by a shift in the phase of an electromagnetic carrier wave with respect to a reference, by an amount corresponding to the symbol being encoded.

Note 1: For example, when encoding bits, the phase shift could be 0° for encoding a "0," and 180° for encoding a "1," or the phase shift could be -90° for "0" and +90° for a "1," thus making the representations for "0" and "1" a total of 180° apart.

Note 2: In PSK systems designed so that the carrier can assume only two different phase angles, each change of phase carries one bit of information, i.e., the bit rate equals the modulation rate. If the number of recognizable phase angles is increased to 4, then 2 bits of information can be encoded into each signal element; likewise, 8 phase angles can encode 3 bits in each signal element. Synonyms biphase modulation, phase-shift signaling.

Source: from Federal Standard 1037C and from MIL-STD-188