Calibration is the operation that establishes the relationship between the values indicated by a measuring instrument and the corresponding values realised by a traceable reference standard.
What it is not
Not adjustment. Calibration measures the error. Adjustment changes the instrument to reduce it. They are often done together and are frequently confused, but a calibration certificate records what was found — and, if adjustment took place, ideally what was found both before and after.
Not verification. Verification confirms an instrument meets stated requirements. Calibration produces the data that verification uses.
Not a pass or fail on its own. A calibration result becomes a pass or fail only when compared against a tolerance you have defined, using a documented decision rule.
What ISO 9001 requires
Clause 7.1.5.2 applies where measurement traceability is a requirement or where the organisation considers it essential. Equipment must be calibrated or verified at specified intervals against traceable standards, identified as to its calibration status, and safeguarded from adjustments that would invalidate results.
There is a requirement that catches people out: if equipment is found to be out of tolerance, you must determine whether previous measurement results were affected and take appropriate action. That can mean revisiting product already released.
Intervals
No standard sets a calibration interval. Annual is convention, not requirement. Intervals should be based on stability, usage, criticality and history — and lengthening an interval is defensible if you have the records to support it.