Is the phasor a time-domain or frequency-domain quantiy?

It is a frequency-domain quantity. In Basic Engineering Circuit Analysis by Irwin, the time domain is written as A*cos(wt+/-THETA) and the frequency domain is written as A*phasor(+/-THETA).

A series of phasor measurements, taken at regular intervals over time, can sometimes be useful when studying systems subject to variations in frequency. The electric power system is one example. The power grid nominally operates at 50Hz (or 60Hz), but the actual frequency is constantly changing around this nominal operating point. In this application, each individual phasor measurement represents a frequency domain quantity but a time series of phasor measurements is analyzed using time-domain techniques. (http://en.wikipedia.org/wiki/Synchrophasor)