Criterion: Rule by which to distinguish one kind of things from another.
Many arguments and decisions depend on some criterion, that may be more or less vague and tacit in many circumstances. Criterions are often formulated in terms of some end, namely whether doing so-and-so would bring about or help to bring about the end.
Two important kinds of criterion in philosophy, both meant as criterions of demarcation, namely between science and non-science, were the criterion of verifiability ("only what is empirically verifiable belongs to science") and the criterion of falsifiability ("only what is empirically falsifiable belongs to science"). Neither criterion is precise, but it does generally make sense to inquire whether a theory is in fact verifiable or falsifiable, and to distrust it if it is neither - because then there is no evidence to support it, nor evidence to refute it, which means that it must be pretty useless if it is a theory about matters of fact.