A painting made straight onto a wall or ceiling while the plaster is still wet. That wet part matters: the colour sinks into the plaster and dries as part of the wall, so the painting cannot be moved or taken down — it IS the building. This is how the famous ceiling paintings in old Italian churches were made. The word names both the technique ('painted in fresco') and any single painting made that way ('a fresco of the sea'). In everyday use, people are a bit loose with it, and you will hear any big old wall painting called a fresco even when the artist actually painted on dry plaster, which is really a different method. Art teachers will correct that; nobody else will. The word came into English from Italian, where fresco simply means 'fresh' or 'cool' — the painter had to work fast, on fresh plaster, before it set.