The word “calibration” may be used (and misused) in different contexts. Here, we are talking about metrological calibration in the world of measurement technology.
Formally, calibration is the documented comparison of the measurement device to be calibrated against a traceable reference device.
The reference standard may be also referred as a “calibrator.” Logically, the reference is more accurate than the device to be calibrated. The reference device should be also calibrated traceably, more on that later on.
With some quantities the reference is not always a device, but can also be for example a mass, mechanical part, physical reference, reference liquid or gas.