A canvas was a piece of fabric popularly used as a painting surface.
In 2269, Spock analyzed several paintings in Flint's possession. He was confused because the canvasses and pigments used were contemporary, but otherwise seemed to be genuine works of long-deceased painters like Leonardo da Vinci. (TOS: "Requiem for Methuselah")
When Ves Alkar tried to convince Jean-Luc Picard that bigger things were at stake in a crisis on Rekag-Seronia in 2369, he figuratively asked Picard to see a "broader canvas" than individual lives. (TNG: "Man Of The People")
According to Onaya in 2372, the writer Revalus compared the use of pen and paper for writers with the use of brush and canvas for painters. (DS9: "The Muse")
The same year, an individual caught in a neural network controlled by The Clown, compared them to being the clown's canvas or blocks of marble. (VOY: "The Thaw")