Datum is the singular form; data is its plural, borrowed from Latin. In practice, though, most people use data as a general, uncountable term, the way you'd use information, and reserve datum for technical fields like surveying. If you're backing up files on your laptop, you're backing up data, not a datum.
The core difference
Datum is one single fact or reference point; data is the collected plural, now often treated as one mass noun.
- data — facts or figures collected for analysis: The scientists spent months collecting data before drawing any conclusions.
- datum — a single known or assumed fact, or a reference point for measurement: Surveyors use a benchmark as their datum when measuring elevation.
How to tell them apart
The word data came into English from Latin as the plural of datum, meaning "a thing given." It started out as a scientific and statistical term before spreading into everyday language with the rise of computing, and along the way many speakers stopped treating it as a plural at all, using it the way they'd use "information" ("the data is ready" rather than "the data are ready").
Datum survives mainly in technical contexts: it can mean a single fact used as a basis for reasoning, or a fixed reference point, such as a benchmark in surveying or a floating marker used in coastal search and rescue to track ocean currents. Outside those fields, it's rare, so if you find yourself reaching for it in general writing, data point is usually the more natural choice.
