(inorganic chemistry) Any oxyanion of actinium, or salt containing such an oxyanion

(medicine, pathology, dermatology) A potentially precancerous skin lesion, typically on the upper portion of the head, characterized by discolored, thickened or scaly growths of keratin and caused by repeated exposure to ultraviolet light, such as from sunlight or tanning beds.

The light available at a point in the atmosphere, especially that available to produce photochemical processes

plural of actinic keratosis