The magnification of a scope is the angle under which an object is in the seen in the eyepiece divided by the angle under which it is seen by the naked eye. (Or would be seen if visible)
For imaging it is wrong to speak about magnification. Rather the (effective) focal length should be mentioned. Sometimes though, even for imaging a magnification factor is mentioned. If so, it should always be mentioned against which base the magnification was given. For example a scope with a focal length of 2500 mm (100") has a "magnification" of 50 as compared with an objective of 50 mm focal length.
The magnification of a scope can be calculated with the following formula:
However, there are limits to the magnification. These are given by the exit pupil diameter (see "Exit Pupil"). The usable magnification range is from 1/6th of the mirror diameter [mm] to 2 times the mirror diameter [mm]. (4-50 times the diameter in Inches) It should be noted that for specific purposes magnifications outside this range are used (see below).
The above formula is valid for the situation where the eye is focused at infinity. It is possible to achieve a different magnification by de-focussing. The exit pupil is then no longer a parallel bundle of light, but a light-cone. For imaging this is called eyepiece projection.
Magnifications with exit pupils larger than 6 mm can be used to find objects. Even though this 'wastes' mirror surface, it is useful since the FOV is larger. Magnifications with exit pupils below 0.5 mm may be used to examine the airy disk, and thus for star-testing a telescope.
I have read somewhere that the best magnification for planetary viewing has an exit pupil of 0.8 mm. (Note: exit pupil = aperture divided by magnification)