When you think about the idea of picture angle, you may be thinking about the angle at which you're shooting from as it relates to the subject. However, this isn't the official definition for picture angle.
Because aspect ratios can be slightly different, depending on the model of camera you're using, a measurement along the diagonal of the shot is often used as an official measurement, removing the discrepancies between models. This is a universal picture angle.
A telephoto lens (such as 300mm) produces a narrower picture angle, and a wide-angle lens (such as 24mm) produces a wider picture angle. The reason that we refer to a 50mm lens as a standard lens is because it produces around the same picture angle as the human eye.