(French, ‘open air’)
The term plein air is used to describe paintings
executed outdoors, as well as the practice of painting or sketching
outside, known as painting en plein air.
Artists began making outdoor sketches in the 17th century,
but the invention of paint tubes in the late 19th century made
it practical to bring paintings to near completion outside.
Painting en plein air became the norm for Impressionists,
especially for Monet, who did most of his painting outdoors.