In 35mm this is the full frame exposed by the camera, with an aspect ratio of 1.33. When the film is projected there is a mask in the projector’s gate to change the aspect ratio to 1.85 or 1.66, cropping the top and bottom of the image. Older films were not shot to be masked and should be projected without a mask. The Academy Aperture is sometimes called the Full Academy Aperture.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission