Chiaroscuro (ke-ära-skur'o):  French: Clair Obscur.

Term from the Italian compound of chiaro (‘light’, ‘clear’) and scuro (‘dark’) used to refer to the distribution of light and dark tones with which the painter...or draughtsman imitates light and shadow; by extension it refers to the variations in light and shade on sculpture and architecture resulting from illumination.

Chiaroscuro has four accepted current usages. [In painting particularly the current definition covers two]:

(1) The gradations in light and dark values of a colour on a figure or object, which produce the illusion of volume and relief as well as the illusion of light and shadow.
 
(2) The distribution of light and dark over the surface of the whole picture, which serves to unify the composition and creates an expressive quality.

The concept of chiaroscuro originated in Italian art theory in the 15th century. Cennino Cennini described the way that painters use gradations of light and dark tones to create the illusion of relief.  In practice, these gradations of light and dark emerged in the 13th century as a means of modelling form, supplanting the medieval techniques (called incidendo and matizando) of laying white and brown or black in linear patterns over a uniformly coloured surface to mark the protrusions and recessions of a relief. Late Gothic panel painters, such as Cimabue and Giotto, achieved gradations of light and dark tones of colour by mixing successively greater amounts of white with pigment to create four to six gradations of a given colour; these would then be applied using the lighter tones to indicate projections and the darker tones to indicate receding parts of figures; edges were blended or overlapped to create gentle transitions from one tone to another. ...a technique of modelling from underneath was frequently employed; in this an underpainting of a dark colour would indicate the shadows, leaving the white of the page or wall to indicate the lights; when [transparent or] semi-transparent layers of coloured pigments were then superimposed, the chiaroscuro underpainting would affect the luminance of the final picture.
 
The technique of modelling from underneath became widespread in oil painting, particularly in the works of Leonardo, who used much black and grey in his underpaintings. The prominence of Leonardo’s example has led to the distinction between ‘modelling in chiaroscuro’, referring to the use of black in the shadows, and ‘modelling in colour’, referring to systems that use darker hues of colour to indicate shadow, as in Cangianti modelling.
 
The use of chiaroscuro as a means to create volume and relief remained characteristic of Western painting until the late 19th century, but the concept of chiaroscuro became broader.

In the Italian Renaissance, it began to signify the imitation of light and shadow in the setting of the picture—not just the luminance gradations of modelling. Leon Battista Alberti identified the reception of light as the third part of painting, advocating that the painter should use white to represent light and black to represent shadow. Leonardo frequently equated chiaro and scuro with light and shadow in his notebooks: ‘il chiaro e lo scuro, cioè il lume et le ombre’. It was in this sense that the words first appeared in print in Baldassare Castiglione’s Il libro del cortegiano (1528), in which he wrote that the painter imitates light and shadow with light and dark: ‘col chiaro, & scuro’.

A few passages by Leonardo also indicate a new way of thinking about chiaroscuro as a single entity rather than as a dichotomy: ‘the chiaro scuro of the shadows’ and ‘the chiaro scuro of a tree’... 
 
In the 17th century, the concept of chiaroscuro expanded to include the organization and distribution of light and dark areas in the overall composition. Charles-Alphonse Du Fresnoy recommended that the painter achieve unity by creating one principal area of light and one principal mass of shadow, with all other lights and shadows subordinate to them in size and intensity. 
 
Most painters renowned for their use of chiaroscuro have made use of strong contrasts and large areas of shadow. Among them are Leonardo, Titian, Jacopo Tintoretto, Caravaggio and most of the tenebrists (see Tenebrism), Rembrandt, Jacques-Louis David, Théodore Gericault and Eugène Delacroix.

At other times in history, however, critics have praised the lighter, natural chiaroscuro of Correggio, Veronese, Nicolas Poussin and Domenichino. Polidoro da Caravaggio is especially known for his chiaroscuros imitating ancient bas-reliefs...
 
A number of critical concepts have been associated with chiaroscuro. Sfumato (It.) is the rendition of blurred, transparent shadows along the contours and edges of interior details, which give the appearance of a veil of smoke.

Unione (It.) refers to the gradual, imperceptible transition at the point where light and shadow come together [fusion]. 

Sweetness (It. dolcezza:) [matching color and light; marrying light and shadow, creating the  the impression of movement and action).  Vasari called this "dolcezza ne' colori unita," the sweet harmony of colors].  Softness (It. morbidezza) and tenderness (It. tenerezza) have been regarded as ideal qualities in the practice of chiaroscuro and refer in various contexts to:

• the transparency of shadows,
• gradual or soft transitions from light to shadow,
• blurred edges off shadow,
• the absence of strong contrasts of light and dark.

Passage (Fr.) describes the placement of a light shadow or half-tone between masses of light, which, instead of separating them, unites them by serving as a smooth passage for the eye.

Extracts and ed. from: Janis Callen Bell: "Chiaroscuro," Grove Art Online. Oxford University Press [January 6, 2006], http://www.GroveArt.Com/