What is calibration? The short answer: calibration is a process of comparison. During calibration, a known uncertainty (often referred to as a standard) is compared to an unknown uncertainty (test equipment). A relationship is established from comparing the measurement data.
When an instrument is tested at a calibration laboratory, the technician will compare the measurement data from the test instrument to a standard. The technician will determine if the test instrument measurements are within tolerance. Adherence to tolerance indicates accuracy. If all measurements are in tolerance, the instrument may be referred to as in calibration. If any of the measurements are outside the stated tolerance, it is commonly said to be out of calibration.
If an instrument is found out of tolerance, ISO/IEC 17025 accredited calibration laboratories will provide measurement data. The data will provide insight into how far out of tolerance the instrument is and at what measurements/points. Measurement data is often shown on the certificate of calibration.
The instrument measurement standard (known certainty) should be traceable to NIST. NIST traceability means that it can be linked back to published standards from the National Institute of Standards and Technology. ISO/IEC 17025 accredited calibration laboratories are required to use standards that are traceable to NIST. This communicates better quality and established accuracy.
Is adjusting an instrument the same as calibration? Not necessarily. When an instrument is adjusted to provide more accurate measurement, it may be said that they instrument is being calibrated. In addition, the term calibration procedure is often used in reference to instructions for adjusting an instrument. Although this is not the technical meaning of the term calibration, it is common phrasing throughout the manufacturing industry.
To minimize confusion, ask for clarification. A quality professional will readily tell you what the term is referencing.