I came across several forums and articles saying that criteria is plural and criterion is singular. Some gave me the impression that criterion is used to denote a set of rules.
What is the correct use of these two words?
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!)