(French: “open air”). Plein-air painting is the practice of painting outdoors, which became more practical  in the 1840s with the introduction of portable easels and oil paints in metallic tubes. It  was pioneered by painters associated with the Barbizon School in France and the Hudson River School in the USA.  Pronunciation here.

Thomas Cole. View from Mount Holyoke, Northampton, Massachusetts, after a Thunderstorm—The Oxbow (detail). Oil on canvas. 1836. Metropolitan Museum of Art, New York.