Calibration is the comparing of a measurement device against to a standard of known and greater accuracy to detect and correct any variation from required performance specifications of measurement device. A standard in a measurement is considered the reference; it is the one in the comparison taken to be the more correct of the two. One calibrates to find out how far the unknown is from the standard.
Calibration, in its true sense, is the comparison of an instrument to a known standard. Proper calibration involves use of a traceable standard: one that has paperwork showing it compares correctly to a chain of standards going back to a master standard maintained by the National and International body.
Calibration refers to the process of determining the relation between the output (or response) of a measuring instrument and the value of the input quantity or attribute, a measurement standard.

Calibration is purely an indication that the instrument was found to be perfuming within the specified specification at the time of the calibration only. It provides confidence that the instrument has been and is operating to manufacturer’ s specifications if it is not calibration house will adjust and inform the owner of adjustment made.
Some people consider calibration a necessary annoyance to keep the auditor off their back. In fact, out of tolerance instruments may give false information leading to unreliable product, customer dissatisfaction and increased warranty costs. In addition, OOT conditions may cause good products to fail tests, which ultimately result in unnecessary rework costs and production delays.

Calibration isn’t a matter of “fine-tuning” your test instruments. Rather, it ensures you can safely and reliably use instruments to get the accurate test results you need. It’s a form of quality assurance. You know the value of testing electrical equipment, or you wouldn’t have test instrumentation to begin with. Just as electrical equipment needs testing, so do your test instruments.
What knocks a digital instrument “out of cal?” First, the major components of test instruments (e.g., voltage references, input dividers, current shunts) can simply shift over time. This shifting is minor and usually harmless if you keep a good calibration schedule, and this shifting is typically what calibration finds and corrects.

Definition: Ability to trace the history, application, or location of an entity by means of recorded identification.
And result of a measurement or related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.

Traceable measurements ensure the uniformity of manufactured products and quality of process.

Remark in the development of technology.

To support equity in trade as well as compliance to regulatory laws & standards.

Assure the users with of the confidence and accuracy of the process.

Validate the whole process.

A parameter, associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measurement.
The estimated amount by which the measured quantity may depart from the true value.

Measurement is the process observing and recording the observations that are collected as part of a research effort and a set of operations that establish, under specified conditions, the relationship between the values of quantities indicated by measuring instrument or measuring system and the corresponding values realized by standards.
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.

Calibration intervals are to be determined by the instrument manufacture recommendations. Commercial calibration laboratories can suggest intervals but in most cases they are not familiar with the details of the instrument’s application.
The intervals are typically based on parameters like type of instrument, drift rates for the various components within the instrument, extent and severity of use. However, when determining calibration intervals as an instrument “owner” several other factors should be taken into consideration such as:
tendency to wear and drift

Manufacturer’s recommendation.

Type of instrument.

Tendency to wear and drift.

Uncertainty of measurement required.

Use of instrument.

Environmental conditions.

Recorded history of maintenance and servicing.

Degree to which the serving personnel are trained.

Trend data obtained from previous calibration record