This tag was originally intended for Procedural Textures, now known as Legacy textures, but has been expanded to include procedural materials introduced with the Cycles render engine and available for both Cycles and EEVEE. Use this tag for questions related to legacy textures or procedural materials.
The Blender 2.79 manual describe Procedural Textures as
Procedural textures are textures that are defined mathematically.
with the removal of the Blender Internal render engine, procedural textures are now called Legacy textures and are slowly being phased out.
Meanwhile Blender introduced a node based materials creation system to accompany the Cycles render engine, and since made available for EEVEE. Procedural materials rely on a mix of image nodes, procedural textures, and other manipulations to create materials.
Use this tag for either legacy procedural textures, or for the use of the node system to create materials for Cycles or EEVEE.