piece of artwork painted or applied directly on a large permanent surface

A mural is a painting on a wall or a ceiling. Murals are usually big. They may be in public places, such as the outside of a building, or indoors.

Ceiling painting
Guernica is a famous Mural of Pablo Picasso. It memorizes the bombing of Guernica, during the Spanish civil war, in 1937.
Copy of a mural, showing the Bisons in Altamira cave. The original murial is over 6.000 years old