Definition: The ratio of the width to the height of an image or screen.
In the 1980s and for cameras, most computer screens and cameras were designed to be 4:3 aspect ratio. Some cameras would only shoot at 4:3 aspect ratio. That is the way the film was designed and produced.
I take many of my own photos and I am by no means a professional. Years ago I developed the habbit of framing or composing my shots so they would fill the screen at the "standard" 4:3 ratio. I have some photos that I really like that were taken in this way.
The picture of Thumb Butte in Prescott featured here was taken by a professional photographer at the 4:3 aspect ratio. I want to feature this photo on my website and would like it to fill the screen. To do so on the new monitors, the photo would be optimized at a ratio that works with my design, say 1900 x 760. This photo would look like this at that ratio.
My original photo showed two major points of interest, the shape of Thumb Butte in the background and the really nice cabin in the foreground. Once a photo is taken and framed for publication at a 4:3 ratio, I do not see a way to fill the screen on today's wide monitors.
I know the answer to this is to take NEW photos in such a way that the photo is wide enough to allow cropping to be adjusted to accommodate the wide format to be publshed or to shown to its optimum for the on-screen publication. I do not see a good way to work with photos that were not taken wide enough to begin with.
Below the image was modified by mirroring a portion of the original image on each side.