A datum is a single piece of information or a fact. Scientists use this word when they collect individual measurements or observations. If a researcher measures the temperature in a classroom at noon, that one temperature reading is a datum. If she measures it every hour for a week, each individual measurement is a datum.

One measurement is a datum, but multiple measurements are data. Most people today use “data” for both singular and plural, but scientists and careful writers still distinguish between them.

You might hear datum used in surveying and mapmaking, where it refers to a reference point from which other measurements are made. Surveyors need to agree on a starting datum, like sea level, so their measurements match up. Without a common datum, one person's map wouldn't align with another's.

While datum sounds formal, it's simply the singular form of a word you already know. When your teacher says “I need more data on reading speeds,” she's talking about multiple individual facts. Each student's reading speed would be one datum, and all of them together form the data. The word reminds us that big collections of information start with single, careful observations.