A more informative and meaningful measure is the return loss of the input port. Instrument makers typically give a worst case return loss over the frequency band of the instrument. In table 1 a range of Return Losses are rated. These are somewhat subjective assessments but the idea is to impart a feel for the units involved. Generally speaking one would like the highest return loss possible.
Some measurements are more sensitive than others and require higher return losses.
Isolation Isolation is best understood by example. Figure 2 shows a typical three port mixer. RF and LO signals are injected at there respective ports. This not desirable undermost circumstances and a perfect Mixer would have no leakage or equivalently infinite isolation. The higher the isolation the lower the leakage.