Traditionally, criteria is plural, and criterion is singular. These reflect the Latin forms. Although most dictionaries and usage authorities still make this distinction, criterion is likely to go the way of datum and agendum (which are only used by small groups of English speakers). That is, criterion will become rarer and rarer, while criteria will become the singular form (with criterias perhaps emerging as the plural).
Already this is happening. In current news publications and popular blogs, for instance, the phrases that criteria and criteria is are far more common than that criterion and criterion is. Criterion still prevails as the singular in searchable books, but this is because books tend to lag behind popular usage by a few years or more, and also because Google’s book-search tool includes lots of scientific and scholarly writing, where Latin forms are conventionally preserved.
Using criteria as a singular noun might still be unsafe when you’re writing for school or in any other formal context, but English almost always eventually imposes its own rules on words derived from other languages, and this appears to be happening as we speak with criteria.
For now, most edited publications still use criterion for the singular noun meaning a standard by which something can be judged—for example:
Williamson implies that’s so, rendering this a meaningless criterion. [Washington Post]
Perhaps one such criterion was for mortgages to have a loan-to-value ratio of 100%. [The Atlantic]
If comfort were the top criterion for selling womenswear, Jimmy Choo would be out of business. [New York Magazine]
Criteria functions as a plural noun:
We’ll look for help today with today’s screen from Morningstar CPMS, which has set up a conservative screen using seven criteria. [The Globe and Mail]
The new criteria have been attacked by many in the industry as being too onerous. [Financial Times]
Although criterion lives, criteria is gaining ground as a singular noun, as used below:
Careful though… this criteria is a double edged sword. [Schriever Air Force Base]
The only criteria is that it must be partially or completely open to the public. [Houston Business Journal]
Dark matter is another criteria that is examined. [Universe Today]