clenu experimental gismu

x1 is an axis/center curve/extended center (or, possibly metaphorically, focus 'point') of x2 in sense x3

For figures of revolution, x3 = "lo ka ce'u srana lo jbislanu". For rotation (in which mass is actually moving around it), x3 = "lo ka ce'u se carna". For a political axis, x3 would be filled with a reference to political importance or alliance. This word should probably not be used for coordinate systems (coordinate axes), but some construct involving manri might be apt for such an idea if used. The axis is the center of all considerations being made and is extended.

In notes:

x1 is a cross-section of object x2 which has an axis, such that the cross-section is made perpendicular to that axis (viewes along it), contains contents x3, and is x4-dimensional.
x1 is a form of ornamentation arranged cylindrically and which is meant to be viewed so that its axis is collapsed to a point (in perpendicular cross-section), which adorns x2