En plein air is a French expression which means "in the open air", and is particularly used to describe the act of painting outdoors.
Artists have long painted outdoors, but in the mid-19th century working in natural light became particularly important to the Barbizon school and Impressionism. The popularity of painting en plein air increased in the 1870s with the introduction of paints in tubes (resembling modern toothpaste tubes). Previously, each painter made their own paints by grinding and mixing dry pigment powders with linseed oil. The Newlyn School in England is considered another major proponent of the technique in the latter 19th century.