Plein-air painting simply means painting outdoors; usually an artist interpreting the scene that surrounds them.
Whether painted off the back of my bicycle or using a folding easel, all of my plein-air paintings are completed outdoors.
In bad weather I might do some of the painting in the shelter of a car, as with the old Dorion train station on a below-freezing morning, but I never leave the engine running.
Plein-air paintings are generally faster and looser than studio paintings, to capture the light and shadows before they move with the sun.