An axis is a straight line, sometimes seen, sometimes not, that is important in mathematics, art, science and our survival. The most famous axis is the one the earth spins around, giving us the 24hour day.
In graphs, one side is usually the "X axis" and the other the "Y axis." Axis lines are also the lines that run through spheres, like our earth, around which those spheres rotate. In a totally different area, axis is used to describe countries in an alliance. In World War II, Germany, Japan and Italy were known as the Axis Powers. More recently, George W. Bush referred to Iraq, Iran, and North Korean as an Axis of Evil.
Primary Meanings of axis
1. 
n 
a straight line through a body or figure that satisfies certain conditions

2. 
n 
a group of countries in special alliance

Full Definitions of axis
1
n a straight line through a body or figure that satisfies certain conditions
 Types:
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coordinate axis
one of the fixed reference lines of a coordinate system

major axis
the longest axis of an ellipse or ellipsoid; passes through the two foci

semimajor axis
onehalf the major axis of an ellipse; the distance from the center of an ellipse to one end

minor axis
the shorter or shortest axis of an ellipse or ellipsoid

semiminor axis
onehalf the minor axis of an ellipse

optic axis, principal axis
a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted

optic axis
in a doubly refracting crystal, the line in the direction of which no double refraction occurs

xaxis
the horizontal axis in a plane coordinate system

yaxis
the vertical axis in a plane coordinate system

zaxis
the third axis in a 3dimensional coordinate system
 Type of:

line
a spatial location defined by a real or imaginary unidimensional extent
n the center around which something rotates
 Synonyms:
 axis of rotation
 Types:
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pin, pivot
axis consisting of a short shaft that supports something that turns

rotor head, rotor shaft
the axis around which the major rotor of a helicopter turns

fulcrum
the pivot about which a lever turns

pintle
a pin or bolt forming the pivot of a hinge
 Type of:

mechanism
device consisting of a piece of machinery; has moving parts that perform some function
n the main stem or central part about which plant organs or plant parts such as branches are arranged
n the 2nd cervical vertebra; serves as a pivot for turning the head
 Synonyms:
 axis vertebra
 Type of:

cervical vertebra, neck bone
one of 7 vertebrae in the human spine located in the neck region
2
n a group of countries in special alliance
 Synonyms:
 bloc
 Types:

scheduled territories, sterling area, sterling bloc
the group of countries whose currencies are tied to the British pound sterling