Calibration is a comparison between measurements - one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device.
The device with the known or assigned correctness is called the standard or Calibrator. The second device is the unit under test, test instrument, or any of several other names for the device being calibrated.
Calibration can be called for :
with a new instrument
when a specified time period is elapsed
when a specified usage (operating hours) has elapsed
when an instrument has had a shock or vibration which potentially may have put it out of calibration
sudden changes in weather
whenever observations appear questionable

In general use, calibration is often regarded as including the process of adjusting the output or indication on a measurement instrument to agree with value of the applied standard, within a specified accuracy. For example, a thermometer could be calibrated so the error of indication or the correction is determined, and adjusted (e.g. via calibration constants) so that it shows the true temperature in Celsius at specific points on the scale. This is the perception of the instrument's end-user. However, very few instruments can be adjusted to exactly match the standards they are compared to. For the vast majority of calibrations, the calibration process is actually the comparison of an unknown to a known and recording the results.

Battery operated with indication
Mains Operated with indication