A test specimen used as a basis for calibrating test equipment or as a comparison when evaluating test results is referred to as a:

A test specimen used for calibrating test equipment or as a benchmark for evaluating test results is referred to as a reference standard. This type of specimen is crucial in various testing environments, serving as a known quantity against which other measurements can be compared.

Reference standards are established to ensure accuracy and reliability in measurements; they provide a basis for validation of the test equipment's performance and ensure that results are consistent across different tests. By using a reference standard, technicians can confirm that their results are accurate and trustworthy, which is essential in maintaining quality control in testing procedures.

In contrast, a null-balancer typically is used in specific types of measurements to balance out signals, a phase shifter alters the phase of a signal, and a high-pass filter allows frequencies higher than a certain cutoff to pass while attenuating lower frequencies. These terms do not apply to the concept of calibration or comparative analysis in the same context as reference standards.