Open air means outdoors, outside of any building or enclosed space. When you eat lunch in the open air, you're sitting outside where the sky is above you and the breeze can reach you. An open-air market is one where vendors sell their goods outside rather than inside a store, with stalls arranged along streets or in plazas.

The phrase captures that feeling of being exposed to the elements: the sun, wind, and weather. Ancient Greek theaters were built in the open air, with stone seats rising up hillsides and no roof overhead. Many concerts and festivals today happen in open-air venues, where thousands of people gather under the sky.

Open air suggests freedom and space. Someone released from a dark cell steps out into the open air and feels the vast difference. When you've been stuck inside all day doing homework, going outside into the open air can feel refreshing and energizing, like your lungs are finally getting the space they need.