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 originally 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. Clarisse supports a subset of OSL features, through its OSL file and OSL script textures. Current Clarisse OSL implementation doesn't support closure and is limited to pattern generation as well as a single OSL execution context per texture node.
More information on OSL is available here: http://opensource.imageworks.com/?p=osl