This definition has been taken from ArchToolbox
Intumescent coatings, often referred to as intumescent paint, are used in buildings as a passive fire resistance measure. They can be applied to structural members as an aesthetically pleasing fireproofing product. Another type of intumescent product is a firestopping pillow or putty that is used to protect openings from fire.
The key feature of intumescents is that they expand significantly when exposed to high temperatures, such as those found in a fire. Some intumescent products can expand to more than 100-times the original thickness. As the product expands it becomes much less dense, which makes it act as in insulator that keeps the high temperatures away from structural members or protected openings.