criterion - noun
A standard, rule, or principle by which to determine the correctness of a judgment or conclusion.
Note: The word criterion is singular. Though criterions is an acceptable plural form, most usage panels and writers use criteria as the plural. The plural criteria, which often appears in the singular in speech, should never be used as a singular in proper writing.
Up to 25 percent of people in whom psychiatrists would currently diagnose depression may only be reacting normally to stressful events such as a divorce or losing a job, according to a new analysis that reexamined how the standard diagnostic criteria are used.
—Shankar Vedantam "Criteria for Depression Are Too Broad, Researchers Say” Washington Post, April 3, 2007