Porthole

A porthole was a type of circular window.

A porthole existed on the inner door of the Doctor's TARDIS during the time when it was flown by the incarnation of the Time Lord who used a Berillian time transformer. Despite no counterpart face of the porthole being visible on the outer door of the Ship, the Doctor was able to look at what was going on outside the TARDIS through this porthole as through a regular ship porthole. He could also still use his sonic screwdriver through the glass. (NOTVALID: Time Is Everything)