A primer (rhymes with “dimmer”) is a first book or introduction to a subject, designed to teach beginners the basics. When you pick up a primer on astronomy, you're getting a book that explains the fundamentals: what stars are, how planets orbit, why we have seasons. Schools once used reading primers to teach children their ABCs and first words.

The word comes from the Latin primus, meaning “first,” and that sense of “first things first” runs through all primers. A chemistry primer won't make you an expert, but it gives you the foundation you need before diving deeper. Think of it as the ground floor of a building: you can't reach the interesting upper levels without starting there.

In another sense entirely, a primer (rhyming with “timer”) is a preparatory coat of paint applied before the final color. Just as a reading primer prepares your mind, paint primer prepares a surface. You might also hear about primer in discussions of explosives or engines, where it refers to something that initiates a larger reaction.

When people say they need “a primer on” something, they're asking for a clear, accessible explanation of the basics, the kind that helps you understand enough to learn more on your own.