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1. The argument of a wave. If the representation of a wave is a function of (κx–ωt), the argument (κx–ωt) is the phase; ωt=temporal phase, κx=spatial phase. 2. The angle of lag or lead of a sine wave with respect to a reference; how far rotation, oscillation, or variation has advanced, considered in relation to a reference or instant of starting. Commonly expressed in angular measure. Phase information, being the measure with respect to the instant of starting, carries the timing information of a seismogram and hence proper phase preservation is of utmost importance. See also phase characteristics, phase response, and compare phasing. 3. In earthquake seismology, an event on a seismogram marking the arrival of a new group of waves, indicated by a change of period or amplitude, or both. 4. A portion of a nonhomogeneous system that is bounded by a surface and may be mechanically separated from the other phases. The three phases of H2O, for example, are ice, water, and steam. 5. A short period of time, as a ‘‘phase of igneous activity.’’