Inert means inactive, motionless, or unable to move or react on its own. A rock sitting on the ground is inert: it won't move unless something pushes it or picks it up. An inert object has no energy or power of its own.

In science, inert has a specific meaning: a substance that doesn't easily react with other chemicals. Inert gases like helium and neon rarely combine with other elements, which makes them useful when you need something that won't cause reactions. That's one reason helium fills party balloons: it's light and doesn't burn.

The word can also describe people who seem lifeless or lacking energy. After running hard in gym class, you might collapse on the grass, lying completely inert until you catch your breath. When someone lounges on the couch all day, barely moving, you might describe them as lying there inertly.

The opposite of inert would be active, reactive, or energetic. While an inert student sits passively during class discussion, an engaged student participates actively.