A camera lens optimized for subjects that are farther away. Compared to a more standard lens, it provides a default image that is more "zoomed in". It does this optically, so there is typically more detail compared to an image captured through a standard lens that has been "zoomed in" digitally.
This is also called a narrower "field of view". For example, if a standard lens has an 80º field of view, a telephoto lens might have a 30º field of view.