Actinic is a term used to describe lighting which produces high levels of light in the blue wavelengths.
This type of light is important in aquarium use as the blue wavelengths of light penetrate water far deeper than other visible wavelengths. Blue light is at the lower end of the light spectrum and is more 'energetic', allowing it to pass through objects further before it is absorbed. Sunlight peaks in the blue areas, because other wavelengths are absorbed as they pass through the atmosphere; the same happens in water (which is why the ocean appears blue) In typical pictures of tropical islands, the surrounding water is a light blue; this is because sunlight travelling through the water is absorbed at different colours or wavelengths, and blue light, being the most energetic, is the least absorbed and hence, most reflected.
Photosynthetic organisms such as plants and algae's are adapted to utilize the blue wavelengths of light, because they are more readily available underwater.
Actinic lighting is designed primarily for marine aquariums, where some corals depend on the algae cells living within them to provide a source of food. The more actinic, or blue light, is produced - the more light is available to the algae.
Actinic light can also be used in freshwater aquariums and is useful for plants, but may produce a slightly unnatural effect. When used in marine aquariums however, actinic light used in conjunction with a bright white light, has quite a pleasing appearance.
Many high-powered lighting systems designed for marine reef aquariums incorporate actinic lights to 'balance' the light output and ensure that corals lower down receive enough light.
Actinic light is different to ultraviolet light, which is even more energetic but outside of the visible spectrum and used for a different purpose.