There are words in English with singular/plural combinations that are a little different from the usual “add an ‘s’ at the end” approach. These words are commonly misused (the singular used for the plural or vice versa). Here are some examples:

criterion (singular): A standard, rule, or test on which a judgment or decision can be based.
criteria (plural): Standards, rules, or tests on which a judgment or decision can be based.

Correct: He has a very specific criterion (fat content) for selecting his lunch.
Incorrect: I have five criterions that I’ll use to judge your performance.
Correct: He has three very important criteria for determining who the agency will be: chemistry, category experience, and creative.
Incorrect: I have five criterias that I’ll use to judge your performance.
Incorrect: I have just one criteria to determine what to do after work.

medium (singular): A channel of communication.
media (plural): Channels of communication.

Correct: Radio is the most-indicated medium for this campaign.
Incorrect: Radio is the most-indicated media for this campaign.
Correct: Radio and Television are the media that will give us the best combined delivery.
Incorrect: Radio and Television are the mediums that will give us the best combined delivery.

datum (singular): A piece of information.
data (plural): A set of information.

Correct: Household Income is the one datum that is missing from this chart.
Incorrect: Household Income is the one data that is missing from this chart.
Correct: Household Income is the one piece of data that is missing from this chart.
Correct: All the data point to the fact that this segment is the correct one to target.
Incorrect: All the datas point to the fact that this segment is the correct one to target.