mentioned in the News Headlines on the Blender website a couple days ago:
Open Shading Language (OSL) is a small but rich language for programmable shading in advanced renderers and other applications, ideal for describing materials, lights, displacement, and pattern generation.
OSL was developed by Sony Pictures Imageworks for use in its in-house renderer used for feature film animation and visual effects. The language specification was developed with input by other visual effects and animation studios who also wish to use it.
OSL is distributed under the “New BSD” license (see the “LICENSE” file that comes with the distribution). In short, you are free to use it in your own applications, whether they are free or commercial, open or proprietary, as well as to modify the OSL code as you desire, provided that you retain the original copyright notices as described in the license.