Jan Vermeer, The Milkmaid, c. 1660, oil on canvas, 18" x 16", Rijksmuseum, Amsterdam, Public Domain via Wikimedia Commons.
Camera Obscura (noun)
A device made of a box with a lens that artists used for centuries to project a perfect, yet upside-down repilca of the subject before it.
One can tell that Jan Vermeer used a camera obscura because the point of view in his paintings are often from the height of a tabletop and his subjects are strongly lit from one side, which was necessary in order for the camera obscura to work well.
