Pure and applied metrology

A few years ago I joined a team of experts writing a revised standard for phasor measurement units (PMUs), devices that seemed to perform amazing feats of measurement in electric power systems. Not long after I joined the group, I began to be troubled by what we were writing about the measurement of frequency. In particular, it seemed to me that the term frequency had not been defined if it was changing, and since the PMU was expected to return a value for the rate of change of frequency (ROCOF), it was obviously expected to be changing. If the term is not defined precisely, I thought, how would you know if the measurement was being made accurately?