A Lightmap is a Texture containing information of light.

The posting of image in Real-time imposes important constraints on the speed of calculation. One of calculation most complex being that of the light, the creators of video games had the idea to calculate this information for all once and to store it in a Texture. This texture will be generally used as multiplier on the matter texture.

The information of light being solidified, it becomes impossible with this technique to modify the parameters of the light. But the visual quality of the scene will be largely improved. That allows very detailed shades (for example when sources of light are placed behind grids, each mesh has its projected shade), but the lights are then fixed and it is impossible to have flickerings like those of the flames, except by again introducing expensive dynamic lights in resources and much more general.

Examples of use of lightmap: http://www.fabrikmen.com/Medias/Showcase#angkor