There are two angles of attack, a geometric angle of attack and an aerodynamic angle of attack.

The geometric angle of attack is the angle between two lines. One of the lines is the chord line of the airfoil which is a straight line between the nose of the leading edge and the trailing edge of the airfoil. The other line is the line of motion of the airfoil through the air.

The aerodynamic angle of attack is measured from the angle of attack at which the coefficient of lift is zero. For symmetrical airfoils, the two angle of attack are the same. For airfoils with mean line cambers, the zero lift angle of attack is negative and the greater the camber the more negative the zero lift angle of attack is.