What term refers to an established requirement or principle by which something can be judged?

The term that best fits the definition of an established requirement or principle by which something can be judged is "criterion." A criterion serves as a specific measure or benchmark that assists in evaluating or making decisions about a particular process, outcome, or performance.

Standards are often broader expectations or specifications used to create a uniform basis for quality or consistency, but they don’t solely focus on the evaluative aspect that a criterion does. Guidelines, while providing general directions or recommendations, lack the precise evaluative nature inherent in a criterion. Protocols are formal procedures or systems set for specific situations, but again, they are not as evaluative as a criterion. Thus, "criterion" is the most accurate term to describe an established requirement or principle utilized for judgement.