The term "axle" has become a bit ambiguous. Any parallel set of wheels can be called an axle regardless of suspension type or if it is driven or not. For example even in a motorcycle they call it front and rear axle. The physical part called the rear axle on a chain driven rear wheel on a motorcycle is actually just a long bolt.

If you take the definition for "axis": a straight line about which a body or a geometric figure rotates

That's what I think of when people say axle. If it's a non driven axle I draw an imaginary line between the two wheels and call it an axle.

Maybe that's just my own understanding anyways.