Definition
Two shafts that share the same centerline of rotation, with one shaft mounted inside or around the other so that both turn about the same axis. The shafts may rotate in the same direction or in opposite directions, and they may turn at the same speed or at different speeds, depending on the gearing or drive arrangement.
Plain English
Two rotating shafts lined up on the same center, one inside the other, each free to spin on its own.
Context Anchor
Seen in turbine engine, propeller, and helicopter drive system descriptions where more than one rotating part must be arranged along the same centerline.
Derivation
From Latin co- meaning 'together' and axis meaning 'axle' or 'centerline.' Coaxial literally means 'sharing the same axis.' Knowing this makes the term self-explanatory: two shafts on one common centerline.
Why Pilots Care
Enables independent control of upper and lower rotors in coaxial helicopter designs while canceling torque reaction.
Analogy
Picture a smaller rod running through the center of a larger hollow tube. Both line up on the same middle line, but they can be separate parts and may turn differently.
Intuition Check
Coaxial does not mean two shafts are merely near each other or connected end-to-end. It means they share the same centerline, often with one shaft inside another.
Example Sentence 1
The Kamov helicopter uses coaxial shafts to drive its two main rotors in opposite directions.
Example Sentence 2
During preflight the mechanic checks the coaxial shafts for signs of wear or misalignment.