Thin lens
Lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the focal length of the lens. Lenses whose thickness is not negligible are sometimes called thick lenses.
Source:Wikipedia en
Thin lens approximation
A thin lens can be reprensented by a infinitely thin lens when we are studying its properties for [lens-object] or [lens-image] distances that are big in comparison to the lens thickness. That is the case in elementary geometrical optics.