What is a principle or standard by which something may be judged?

The term that best represents a principle or standard by which something may be judged is "criteria." Criteria establish a benchmark against which evaluation or assessment can occur, allowing for a systematic approach to determine how well something meets specific standards or requirements. In many contexts, such as in assessments, quality control, or evaluations, criteria serve as the basis for making comparisons and decisions.

In contrast, while "standard," "protocol," and "guideline" all relate to rules or frameworks, they do not capture the essence of a judgment mechanism as effectively as criteria does. A standard typically denotes a level of quality or attainment rather than an evaluative framework. A protocol outlines procedures rather than criteria for judgment, and guidelines provide recommendations without necessarily setting firm standards for evaluation. Thus, criteria serve the specific purpose of enabling judgment and assessment, making it the most appropriate choice for the question asked.