An extended test of the performance characteristics of a clock or oscillator. A characterization involves more work than a typical calibration. The device under test is usually measured for a long period of time (days or weeks), and sometimes, a series of measurements is made under different environmental conditions. A characterization is often used to determine the types of noise that limit the uncertainty of the measurement and the sensitivity of the device to environmental changes.
Sources:
NIST IR 8323r1
from
NIST T&F Glossary