Maybe it is easier if you think of pixels rather than the absolute size of the image. The expression "100% crop" is by itself not very clear but there is a consensus of the meaning of it.Hmmmm. I guess I understand what you're saying, but 1:1 looks different on different monitors, doesn't it? So isn't it a kind of poor universal unit of measurement? Like I said, I'm using a 1920x1200 monitor. So 1:1 magnification on my monitor is something like 1456x1120 for an M43 shot. If I were to use a 1650x1080 monitor, wouldn't the 1:1 magnification be a smaller area of the picture?
My G1 images are 3000x4000 pixels. They don't fit the screen. Whenever an application shows me the whole image it has been interpolated to a lesser dimension, counting the pixels.
Most image editors can be set to show the image at full size = 100% size = natural size = native size or whatever they call it. In Photoshop this is done by, for example, setting the image to 100% in the little Navigator window, or by choosing View-->Actual Pixels. Then one has to scroll around to see all parts of the image. Now every single pixel in the image corresponds to a single pixel on the monitor. That is the largest "magnification" that ever is useful for judging details in an image (pixel peeping indeed). If we set the image to more than 100% size it is enlarged and the image viewer interpolates what pixels between the native 12.000.000 should look like, similar to what it does when it shows a smaller version of the image.
So, sometimes we want to see a 100% crop. That is a part of the image only (saves bandwidth and downloading time), but we can see it just as when the image is set to be viewed in 100% size.
Look at my dirty B&W image above. It's resized, it doesn't tell us much about details. Before processing it I set it to 100% and marked a part of interest using the Rectangular Marquee Tool in Photoshop and copied that part to a new image. That is the color version of the girl's eye. A 100% crop. I could have marked a bigger or smaller part of her face, it still would have been a 100% crop.
I'm sorry if I explained things you already know, and for repeating what Jonathon already said, but I hope you now have the same idea about what a 100% crop is as everybody else.
regards,
/Jonas