A film image with a greater aspect ratio than the ordinary 35 millimeter frame.
The aspect ratio of a standard 35 millimeter frame is around 1.35:1, although cameramen may use only the part of the frame which will be visible on a television screen (which is 1.25:1). Viewfinders are typically inscribed with a number of frame guides, for various ratios.
Note that aspect ratio refers here to the projected image. There are various ways of producing a widescreen image of any given proportion.
Cinemascope, for example, required anamorphic lenses that put a distorted image into a standard frame and anamorphic projection lenses to unsqueeze the image and spread it over the wide screen. But competing companies produced widescreen images in the same ratio merely by masking part of the frames taken by ordinary cameras--sometimes using a "hard matte" in the camera itself, sometimes by specifying projector mattes.
A low-budget movie called Secret File: Hollywood, often ridiculed as a collection of bloopers, is actually an example of a film that is always projected wrong. All the lights and microphone booms visible above the actors should be concealed by a projection matte, creating an image that would fill a wide screen for little money.
There are also widescreen processes that use larger film frames: 65mm, 70 mm. These can increase both the height and the width of the image, as well as allowing for a finer picture.