The proportions of the frame. In 16mm and 35mm the camera photographs a slightly square image, with an aspect ratio of 1.33 to 1. Aspect Ratios are usually shorted to leave out the “- to 1,” taking for granted that it will always be in relation to 1, an so “1.33 to 1” can just be called “1.33” In 35mm 1.33 is known as the Academy Aperture. In 35mm the image is usually shot with the Academy Aperture and then masked in the projector to produce a wider image: 1.85 in the U.S. and 1.66 in Europe.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission