From AMS Glossary
(Also called coherent averaging or time-domain averaging.) The time-domain integration of measurements in a coherent radar over a sequence of pulses or over an observation interval, prior to estimating the signal properties, to improve the signal-to-noise ratio while minimizing signal processing.
For such integration to be effective, the integration period must be limited to the time over which the phase of the signal relative to a reference phase does not change substantially (i.e., the coherence time of the signal). The effect of the coherent integration process is to reduce the effective data sampling rate and the Nyquist frequency.