Plein air is a French term meaning "full air", or "open air".
In practice it means standing outside and painting directly from life. It's very
challenging because you have to work quickly and accurately despite distractions
and rapid changes in the environment, such as the movement of the sun and speeding
cars. One of the great things about plein air painting is that it's a perfect
excuse to hang out in the shade of a big tree on a hot summer day.