From the italian word "a fresco" = fresh

This term indicates the unique pictorial technique, where no agglutinants (egg, oil,
glue, acrylic resin, etc.) is used to fix pigments to the painted support.

The pigments mixed with water, are deposited on a mortar, extinct lime,
freshly applied on a wall. The lime in contact with the air, undergoes
a transformation called carbonatation, which creates a fine sheet of limestone,
and trap the pigments, fixing them in a durable way to the support.

Among the great Masters of painting a fresco, quote Giotto,
Piero della Francesca, Fra Angelico and Michel-Angelo.