En plein air is French for "in the open air." In painting, it means working outdoors, in front of the subject, instead of in the studio. The painter sets up an easel in a field, on a beach or at a window, and paints the light, the weather and the colors as they are at that hour, before they change.
In French the phrase simply describes anything done outdoors. English speakers kept it unchanged for painting, and today it is often shortened to plein air, both for the practice and for the pictures it produces.
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Before 1800, a landscape was usually composed in the studio. The painter drew and took notes outdoors, then went back indoors and built the picture from memory and sketches, placing trees, ruins and clouds where the composition needed them. Those sketches were guides for the painting, not meant for exhibition.
That habit began to shift in Italy in the decades around 1800. Painters from France, Germany, Britain and Scandinavia went south and worked outdoors with small sheets of paper and oil paint, fast enough to catch a sky before it moved.
The French landscape painter Pierre-Henri de Valenciennes turned what these painters were doing into a method. In a treatise published in 1800, he urged young painters to work in nature and to use the oil sketch to capture changing conditions of atmosphere and weather, before composing the finished picture in the studio.
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