An emissive map is a texture that does not react to light, so the pixels are shown at full intensity. It is also referred to as a Fullbright Map, Glow Map, Incandescence Map, or Self-Illumination Map. This is used for a 'glow in the dark' effect, but does not emit light onto adjacent surfaces.
This can be used alongside a Diffuse map to add a glow effect, such as magic runes on a sword or the heated material on a torch.
Below is the emissive texture used in the above example: