What is calibration?

The demonstration that a particular instrument or device produces results within specified limits by comparison with results produced by a reference or traceable standard over an appropriate range of measurements.

It means that calibration is a comparison of the results produced by the instrument or device with the results produced by the reference device or standard results.

For example pH meter and conductivity meters are calibrated by comparing the produced results with standard results while thermometer and pressure gauges are calibrated by comparing the results produced by the reference device.