Term for errors that are constant for a measurement
The term I would search for in literature is drift, meaning a very slow change in parameters, often due to the mechanisms described by Andy aka (temperature and aging).  The opposite term to search for would be stable or stability.

For example, you could say something like:


"the amplitude error stability is better than 1 mV over 1 hour"
"the phase offset drifts by 0.01°/day"


A couple of related terms are:


Repeatability refers to errors due to repeated runs on the same device under test using the same measurement equipment.
Reproducibility refers to errors measuring the same device using different sets of measurement equipment.


Your measurements probably have some error due to drift in the device under test, but also some repeatability error due to drift in your equipment.