A mixed-light exposure is one that contains natural light and artificial light. An example would be a wildlife photo shot with ambient light, but with a flash strobe used to add fill light.
When using mixed-light in nature photography, the ambient light is usually the primary light source, with artificial light being used for fill, highlight, or creative purposes. For fine art nature photography, care should be taken to ensure that the artificial light is not too harsh or distracting, and that it has the proper color temperature in relation to the ambient light. It should also be used judiciously with wildlife, with careful consideration of the potentially negative effects and stress it may cause.