No, I don't mean a stable fire.
A blaze is a broad white stripe down a horse's face. To be called a blaze, it has to reach at least the inside of the eyes and in some cases may cover the eyes. (If it does, the eyes are often blue).
This Quarter Horse mare has a blaze which runs all the way down to the lips. The lower lip also has some white on it. Note that the skin of the nose is pink.
Very wide blazes are sometimes associated with pinto genes, but as you can see this horse is otherwise solid.