I. ˈplān- noun
Etymology: French, literally, full air
: outdoor daylight
to paint in plein air
to render the high tonal values of the local plein air
II. adjective
1. : relating to the method, action, or product of painting in outdoor daylight
2. : of, relating to, or constituting a French art movement starting about 1865 which attempted to represent effects of outdoor light and air not observable in the studio