I've been reading a camera review which shows a picture with a tiny rectangle highlighted. It then shows that rectangle expanded to the same size as the original picture so that one can see the detail. This is referred to as a 100% crop in the review but that does not agree with the definitions given here. So what is it? I would call it a sectional enlargement!
"100% crop" means a crop of the image at 100% enlargement (i.e. not scaled down to fit on the screen).
I agree the term is totally misleading - it sounds like a 50% crop should be half an image, so 100% crop should be the whole thing!
It's a term that's widely used unfortunately, I prefer the term "actual pixels" so I use that wherever possible in the hope that it will catch on.
When viewing a 100% crop you are viewing each pixel at 100% size. In other words each pixel in the photo gets an entire pixel on your computer screen or other viewing screen.
When an entire high resolution photo is viewed on most computer monitors or other smaller screens the image must be scaled down to fit. There isn't a standard term for this because not all cameras output at the same resolution and not all viewing devices use the same resolution. So the amount of scaling varies significantly depending on the resolution of the photo and the resolution of the viewing device. If you are viewing an image from a Canon 5DII (5616x3744) on an HD monitor (1920x1080) using the entire screen the photo must be reduced by a factor of 3.47 to fit the entire height of the image on the screen. That means the image is being viewed at a magnification of 0.28x, or at 28%. Each pixel on the screen is displaying the information from a roughly 3.5x3.5 pixel square of the image. But if I take an image from my Canon 7D (5184x3456) and view it on the screen of my 320x240 pixel mobile device, it is being scaled to approximately 6.2% to fit. Each pixel on the screen is combining information from a 16x16 square made up of 256 pixels in the original image!