The relationship between time and intensity on a dimmer. (ex. a linear output would mean that 10% through a fade, the dimmer would be at 10% as well). Since this configuration doesn't translate into either a perceived smooth fade or a measured one, modern lighting boards have dimmer profiles / programable curves that can stipulate what level a dimmer should be outputting at a certain % through the fade.
Example of a standard "S-Curve."
Example of a standard "S-Curve."