Vital parameters of regulator quality:
1. The output voltage's temperature coefficient of is the change in output voltage with temperature (perhaps averaged over a certain temperature range), while...
2. Initial accuracy of a voltage regulator (or basically "the voltage accuracy") reflects the error in output voltage for a fixed regulator with no taking into account temperature or aging effects on output accuracy.
3. Dropout voltage - the minimum variation between input voltage and output voltage for which the regulator can still supply the fixed current. A Low Drop-Out (LDO) regulator is considered to work well even with an input supply only a Volt or thus above the output voltage.
4. Absolute maximum ratings are described for regulator components, specifying the constant and peak output currents which may be employed (sometimes limited internally), the maximum input voltage, maximum power dissipation at a specific temperature, etc.
5. Output noise (thermal white noise) and output dynamic impedance might be specified as graphs versus frequency, whereas output ripple noise (mains "hum" or switch-mode "hash" noise) may be given as peak-to-peak or RMS voltages, or as their spectra.
6. In a regulator circuit Quiescent current is the current drawn internally, not available to the load, generally measured as the input current when no load is connected (and therefore a source of inefficiency; some linear regulators are, surprisingly, more efficient at extremely low current loads than switch-mode designs due to this).