The additional points introduced at a fixed distance from the center point of a two-level factorial design, in order to create a Central Composite Design (CCD). CCDs are a type of response surface design. Axial points are also called ‘star’ points. Axial points in a design are used to estimate the effects of curvature in the response surface, such as quadratic and cubic effects. There are always twice as many axial points in a Central Composite design as there are Factors.
Central Composite Designs from NIST: - http://www.itl.nist.gov/div898/handbook/pri/section3/pri3361.htm