The technical definition of a telephoto lens signifies a lens that has a focal length that is longer than the actual barrel length of the lens. There are other lenses which can also focus further away but are not telephoto because they don’t have a long focal length.
Additionally, they are extremely long and thus unwieldy. Please note focal length has nothing to do with the barrel length. These are two different things.
The basic purpose of using a telephoto lens is to bring a subject that is far away from the camera closer to it. Telephoto lenses usually have a focal length that is in excess of 85mm. A thing that characterizes telephoto lenses is their angle of coverage and the corresponding angle of view.
The longer the focal length of the lens the shorter is the angle of view. A Canon EF 100-400mm f/4.5 – 5.6 lens, e.g., has an angle of view of 24 ◦ to 6 ◦ 10’. On the other hand, a Canon EF 70 – 200mm lens has an angle of view of 34 ◦ to 12 ◦.