"Criteria" versus "criterion"

Solution 1:

According to Merriam-Webster.com, criteria is indeed plural and criterion is singular. The dictionary definition is "a standard on which a judgement or decision may be made"; it's often used synonymously with "requirement(s)", as in "if you don't meet the criteria you won't be allowed in" or "I have one all-important criterion by which I judge potential plumbing contractors: is their shirt tucked in to their belted pants?"

M-W.com also notes that the use of "criteria" as singular has been gaining ground for a long time (50 years or so) and may be considered acceptable usage by now.

(To which I say, fight the change! Use "criterion" correctly!)

Solution 2:

Criterion comes from Classical Greek. Its gender is neuter and it belongs to the second declension — like the word phenomenon.