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 24-hour 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 Korea as an Axis of Evil.