Portal:OpenGL Shading Language/Built-in GLSL Stuff
GLSL provides a number of built-in variables and functions. Most of the variables are specific to a certain stage, providing stage-specific information to that shader. Most of the functions are not stage specific; they can (usually) be called from any shader stage.
All built-in GLSL variables begin with the "gl_" prefix, which is reserved in GLSL for this purpose.