Is it ensure, insure, or assure?
1540s, "imaginary straight line around which a body (such as the Earth) rotates," from Latin axis "axle, pivot, axis of the earth or sky," from PIE *aks- "axis" (cf. Old English eax, Old High German ahsa "axle;" Greek axon "axis, axle, wagon;" Sanskrit aksah "an axle, axis, beam of a balance;" Lithuanian aszis "axle"). Figurative sense in world history of "alliance between Germany and Italy" (later extended unetymologically to include Japan) is from 1936. Original reference was to a "Rome-Berlin axis" in central Europe. The word later was used in reference to a London-Washington axis (World War II) and a Moscow-Peking axis (early Cold War).
axis ax·is (āk'sĭs)
n. pl. ax·es (āk'sēz')
A real or imaginary straight line about which a body or geometric object rotates or may be conceived to rotate.
A center line to which parts of a structure or body may be referred.
The second cervical vertebra. Also called epistropheus, vertebra dentata.
An artery that divides into many branches at its origin.
Plural axes (āk'sēz')