Criteria is a noun which represents any representative standards, rules, or measures by which a particular issue may be decided.
Criteria also can represent any standard by which a particular item can be evaluated, or by which performance may be evaluated. The varieties and types of criteria that may exist when judging an issue vary wildly by the type of item that is being considered. For example, in athletic competition, the judging staff may enact or enlist a pre-determined set of criteria by which to determine who has successfully completed certain elements of the competition as opposed to others who may in fact be lacking.
Criteria may also be a factor when assigned a project. Certain criteria may be outlined up from that dictate not only the way in which the activity is performed but also the way in which the deliverables will be measured.
This term is defined in the 3rd and the 4th edition of the PMBOK.