What are these windows/portholes on the English Electric Canberra bomber?
It's called a "Direct Vision" (DV) window. The canopies are sealed shut, so the small DV window is provided as a way to open a small hole if required. For example, to communicate on the ground or in the event of icing or other canopy issues.

Edit: I wasn't able to find a formal source, but this page is fairly authoritative:

http://alverstokeaviation.blogspot.com/2016/08/english-electric-canberra-technical-tour.html