Fresco

Etymology

Italian "fresco" (fresh, cool), borrowed from a Germanic source and a cousin of English "fresh." The full phrase was "affresco" or "a fresco" — "on the fresh (plaster)" — because the paint goes on while the plaster is still damp.

noun · /FRES-koh/
A wall painting made by applying pigment onto fresh, wet plaster so the color fuses into the wall as it dries.
What it names
Both a technique and its unforgiving discipline — you must paint fast, before the plaster sets — which explains the durability and scale of works like the Sistine Chapel ceiling.
Example
Because it is a true fresco, the color is not on the wall so much as in it.
Related idea
A fresco binds pigment into wet plaster and becomes part of the wall; a mural is any large wall painting, including ones simply painted onto a dry, finished surface. Every fresco is a mural, but a mural need not be fresco.