Sfumato is an Italian term used in oil painting to describe the skillful blending of colors and tones such that they seem to melt into one another imperceptibly. It is often applied to the work of Leonardo da Vinci who was one of the first identified artists to use the atmospheric technique.
Definition: In oil painting, the creation of soft, delicately blended effects by the fusion of one tone into another, particularly in glazes. (*)
The technique was perfected during the renaissance period and would be employed by other artists such as Fra Bartolommeo (1472-1517), Giorgione (1477-1510), Corregio (1489-1534), and Federico Barroci (1528-1612).
(*) Mayer, Ralph. A Dictionary of Art Terms and Techniques, NY, HarperPerennial, 1981.