En plein air is a French expression meaning "in the open air." This is most often in reference to painting outdoors. Artists have painted outdoors forever, but the mid-19th century impressionists such as Claude Monet or Pierre-Auguste Renoir brought plein air painting en vogue. The release of tube paint later that century increased the popularity of plein air painting.
Today plein air painting is still en vogue with many artists and numerous plein air painting groups and networks such as the Michigan Plein-Air Painters.