4:3

4:3 refers to an Aspect ratio (AR) with a height that's 3/4 the width. It's also commonly referred to as 1.33:1 or simply 1.33, in reference to the width being approximately 1.33 times the height. This AR is also commonly referred to as fullscreen, referring to the fact that video with this Aspect Ratio fills the full screen of an analog TV. Widescreen video, on the other hand, must be letterboxed, or vertically compressed to be viewed properly on a 4:3 TV, resulting in only part of the screen being used for the image.

Academy Ratio
In addition to TVs some films, including most shot before the 1950s, have an AR very close to 4:3 of 1.37. Film with this Aspect Ratio is commonly referred to as fullscreen despite having a slightly different AR than fullscreen video for a TV. Fullscreen film can either have the sides cropped or be displayed with a very slight AR error of around 3%.