It is a term used in HDR --High Dynamic Range-- processing.
Yes, although it is not restricted to that.
In general terms, one can look at an image as a recording of a range of luminances that existed in the original scene. Some scenes have low contrast (subject contrast multiplied by lighting contrast), while others span a huge range of different luminances.
Ideally, for a realistic rendering of the scene, one would like to recreate the same range of luminances when we look at the resulting images, be it on screen or in print. However, since the display modalities do not allow to produce arbitrary amounts of luminance, we'll have to compromise. We'll have to apply a tonemapping function to translate from input luminances to output luminances.
Printed output is typically limited to allow an up to 100:1 ratio, displays on average range from say 80:1 to 500:1 (sometimes even higher).
So what to do when we have an original scene with a contrast range of say 50:1, and we want to print it on a glossy paper with a 100:1 capability? Well, to reproduce the original contrast sensation, we can print it at the original scene's contrast, but we can print it a bit darker or lighter. It will still fit in the capabilities of the medium but the mood changes. We can also choose to transform the original scene to a more contrasty one, not as realistic anymore but with more punch. We then stretch the range of luminances to cover the full capability of the paper. This is typically done by adjusting the black point and white point (with a levels adjustment on the histogram).
We can even choose to increase the contrast for some tones, and/or reduce the contrast for others, or to brighten some tones and darken others. This is typically done with a curves adjustment or burning and dodging techniques. While this will violate a realistic reproduction, it does allow to emphasize certain structures in a more creative fashion.
Now, what to do when the original scene exceeds the capabilities of the paper. First of all, we may have a problem with recording such a contrast range. Technically, most DSLRs are limited to recording 4095: ratios, some can theoretically reach a 65535:1 ratio. Seems enough one would guess, but there are many scenes that exceed such a range, and shadows in such large ranges have a lot of noise, so one tends not to push the envelope to the maximum capabilities. The HDR techniques specifically address these issues and can record much higher ratios.
Let's assume we have a scene with a 2000:1 contrast (e.g. fill flash was used to improve the shadows). The straight forward way of dealing with it, and be able to print it, is to scale down the contrast to fit the 100:1 paper capabilities. Technically not difficult, with a linear tonecurve and a Levels adjustment, but it will result in a low contrast and dull looking image.
This is the most common scenario (HDR is an extreme version of it) requiring a non-linear tonemapping function to kind of shoehorn the large range into what we can achieve in output, while still maintaining the look of the original scene. This is also an interesting field in imaging research, and it allows a lot of creativity.
The tonemapping function that allows to maintain the original contrast
sensation is scene dependent, highly non-linear (it rather is adaptive to local contrast throughout the image), and also has to do with the
human visual system and
perception. And then there is also the creative aspect where one deliberately
wants to deviate from a
perceptually realistic rendering, e.g. to emphasize certain aspects of the image.
So we end up with tools and techniques that allow us to map the different tones in the original scene to tones in the output (AKA tonemapping), sometimes be cause we have to, but hopefully more often because we want to improve the output quality. This also implies that one could/should adapt the transfer function to the output modality's capabilities, and the viewing conditions.
Bart