If you want the dictionary definition, just double click on any word.

This isn't so much a common mistake as a common cause for arguments (as is often the case with words of Latin origin).

The dictionaries treat data as a group noun, meaning information, especially facts or numbers, collected for examination and consideration and used to help decision-making, or meaning information in an electronic form that can be stored and processed by a computer.

Then they go on to confuse matters by giving the following kind of example:-

The data was/were reviewed before publishing.

So, which is it, was or were? Strictly speaking 'datum' is the singular form and 'data' is the plural form.

If you're writing for an academic audience, particularly in the sciences, "data" takes a plural verb.

For example:-

The data are correct.

But most people treat 'data' as a singular noun, especially when talking about computers etc.

For example:-

The data is being transferred from my computer to yours.

And I have to be honest, I've never heard anyone ask for a datum.