Elementary means basic, fundamental, or relating to the first principles of something. When a detective calls a problem elementary, she means it's simple and straightforward once you understand the basics. Elementary math includes addition, subtraction, multiplication, and division: the foundations you need before tackling algebra or calculus.

An elementary school teaches children these fundamental skills: reading, writing, basic math, and core knowledge about the world. Elementary students are typically between five and eleven years old, building the foundation for everything they'll learn later.

The word suggests something is at the beginning level, but that doesn't mean it's unimportant. Elementary knowledge forms the base that everything else builds upon. A pianist might practice elementary scales every day, even after becoming an expert, because those basics matter. Scientists talk about elementary particles, the smallest building blocks of matter, because you can't break them down into anything simpler.

When someone says “that's elementary,” they mean it follows naturally from what you already know. If you understand that 5 + 5 = 10, then knowing that 10 - 5 = 5 is elementary: it's the logical flip side of what you've already learned.