Sfumato

Sfumato is a term coined by Leonardo da Vinci to refer to a painting technique which overlays translucent layers of colors to create perceptions of depth, volume and form. In Italian sfumato means "blended" with connotations of "smoky".

2. the technique of blurring or softening sharp outlines by subtle and gradual blending (feathering) of one tone into another. The smokelike haziness of this effect slightly lessens the perception that a still image is entirely still, instead lending a vague sense of movement. It is best known in the paintings of the Italian Renaissance artists Leonardo da Vinci (1452-1519) and Correggio (Antonio Allegri) (Italian, 1489-1534).