About this term
Source: Oxford University Press
Type of painting medium used to bind pigments. Strictly, and as used by Cennino Cennini in his Libro dell’arte, the term may be applied to any combination of pigment and medium. By the 16th century, however, Vasari used the term specifically to mean egg tempera, a paint made using egg yolk as the binding medium, and this is the meaning now generally understood.
From Grove Art Online