As a noun: A bunch of color (usually paint) applied onto a canvas (which resembles a white, thick piece of fabric attached to a frame). Depending on the skill of the artist and/or the taste of the viewer, it can be labeled as 'art'... Or an over-priced piece of wood, fabric, and paint.
Verb: The act of applying color (usually paint by the use of a brush) over the canvas.
Noun: The Mona Lisa is a painting which became famous because someone stole it... If you've ever been to the museum, you can see WHY it was stolen. (It's the ONLY painting that could possibly fit through the doorway. The other paintings are HUGE.)
Verb: Person A: What the hell are you doing?
Person B: Painting.
Person A: ... (Does dumping buckets of paint on a canvas count as 'painting'?)