Painting in water-color on a freshly laid wet surface of absorbent plaster.
In true fresco (buon fresco), the earth pigments are mixed with water of hydrate of lime on a wet surface; in dry fresco (fresco secco), the dry surface is wetted with water only just before touching.
Real frescoes adorned the walls of prehistoric caves, the palaces of Mycenre and Tiryns (1500 B.C.), and were used by the Egyptians, Greeks, Romans, and early Christians in the catacombs.
Fresco painting was revived again in the 12th and 13th centuries under Giotto, Cimabue, Orcagna, and later by Michelangelo, Pinturicchio, Leonardo da Vinci, Guido Reni, and the modern Puvis de Chavannes.
New Catholic Dictionary