Criterion is singular and criteria is plural. It is generally regarded as incorrect to use criteria as a singular noun (with criterias as a bogus plural), though it is commonly seen and heard in the print and electronic media, and in some law contexts as well. 

The phrase set of criteria is often used when a singular expression is required. 

Check out these expressions:

What are the criteria?

What is the criterion?

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Incident refers to something that happens, especially a single event. 

Incidence is the rate of occurrence of something: the frequency with which something occurs.

Do not be tempted to use incidence when what you mean is incident.