The angle which a straight line makes with the positive direction of the x-axis measured in the anticlockwise direction is called the inclination (or angle of inclination) of the line. The inclination is usually denoted by θ.
If θ is the inclination of a line l, then tan θ is called the slope or gradient of the line l. The slope of a line whose inclination is 90° is not defined.
The slope of the x-axis is zero and the slope of the y-axis is not defined.