Plain English Meaning
Two shafts mounted one inside the other, sharing the same center, so each can spin at its own speed without affecting the other.
Full Definition
A shaft arrangement in which one shaft rotates inside another along a shared center axis, allowing two rotating assemblies to turn independently at different speeds on the same centerline. In a gas turbine engine, concentric shafts connect separate compressor and turbine stages so each spool can rotate at its own optimum speed.
Context Anchor
Seen in gas turbine engine descriptions, especially when the handbook explains how different compressor and turbine sections are connected inside the engine.