usage note for criterion

Like some other nouns borrowed from the Greek, criterion has both a Greek plural, criteria, and a plural formed on the English pattern, criterions. However, the -s plural is rarely used; the plural ending in -a is the usual form: These are the criteria for the selection of candidates. Though criteria is properly a plural noun, it is increasingly used as a singular noun, most often in speech but also occasionally in edited prose: One criteria is that the candidate must be over 18. This use of criteria as a singular noun is generally considered incorrect.