Originally Posted by Polaris OBark

The datum is a single measurement.
The data are a set of measurements.
The data-set is a collection of measurements.

Many people use "data" interchangeably with "data-set". Since we aren't writing in Latin, a little bit of flexibility is appropriate.

British people say stuff like "the crowd are going wild." It isn't grammatically wrong. It is purely convention. (They invented the English language, so it is very hard to make the case that it is grammatically or in any other way incorrect.)