Coaxial describes something that shares the same central axis or line. Imagine two circles, one inside the other, both centered on the exact same point, like a target with its bullseye. Those circles are coaxial because they share the same center line.

An axis is an imaginary line that something spins around, like the rod through the center of a spinning top.

You'll most often hear this word when people talk about coaxial cables, the thick round cables that connect televisions to cable boxes or antennas. These cables are called coaxial because they have a wire running down the very center, surrounded by insulation, then another wire layer wrapped around that, all sharing the same central axis. This design protects the signal traveling through the inner wire from interference.

Coaxial rotors on some helicopters work the same way: two sets of blades spin in opposite directions around the same central shaft, one above the other. Engineers also design coaxial gears and coaxial machine parts when they need multiple pieces rotating around the same center point. The key idea is always the same: multiple parts, one shared center line.