operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication
In plain terms, you compare an instrument to a traceable standard and make a determination from the results.
provision of objective evidence that a given item fulfills specified requirements
NOTE 1 When applicable, measurement uncertainty should be taken into consideration.
NOTE 5 Verification should not be confused with calibration. Not every verification is a validation
What's the difference? A bit formal in that calibration speaks towards measurement traceability and includes measurement uncertainties. Verification leads to more of a pass / fail type of result.
Of course, they had to throw in validation:
verification, where the specified requirements are adequate for an intended use