The space between two lines or surfaces at the point at which they touch each other
Angle conversion rates
1 Degree = π/180 Radians
1 Gradian = 0.9 Degrees
1 Minute of arc = 1/60 Degrees
1 Second of arc = 1/3600 Degrees
The space between two lines at their intersection is one instance of when an angle can be measured. Certainly the most common measurement of angle is the degree. Separating a circle into 360 individual sectors creates 360 one degree sectors. Hence, this method of measurement is easy to understand as many people instinctively know that 90 degrees are in a right angle, through which 360 degrees in a circle can be derived.
The more mathematical, and also the SI derived unit of, measurement is the radian. A circle consists of of 2π radians, therefore providing a figure of ~ 57.3 degrees for 1 radian.
By comparison, the gradian appeared in France as the angular unit for the new metric system of the late 18th century. It divides the circle into 400 parts, rather than the 360 of the degree. It is useful as right angles occur at 100 gradians, and so the four quadrants are easily distinguishable. Furthermore, when the meter was defined as 1/10,000,000 of the distance between the North Pole and the equator, through Paris, 1 gradian equaled 100 kilometers of distance along the meridian.