Datum and Data

Datum (n.): something given or admitted especially as a basis for reasoning or inference; something used as a basis for calculating or measuring.

Data (n.): factual information used as a basis for reasoning, discussion, or calculation; information output by a sensing device or organ that includes both useful and irrelevant or redundant information and must be processed to be meaningful; information in numerical form that can be digitally transmitted or processed.

Datum comes from the neuter form of the Latin word datus, "given." A datum is a given piece of information. Since the neuter plural ends in -a, the plural of datum is data. However, in today's society we most often use data as a singular word. It is still grammatically correct to say, "These data are," although it sounds strange. But we usually say, "This data is."