Refers to cold air or a chilly atmosphere, often with a slightly literary or descriptive feel. Can be used for cold weather, a cold draft, or the coldness of a place.
Cold air flowed into the room.
A chill hung over the mountaintop.
冷気 is cold air, the direct opposite of the hot, oppressive air described by 熱気.
Compound of 冷 (rei, 'cold') and 気 (ki, 'air, atmosphere'). The reading is on'yomi, and the word is a Sino-Japanese compound.