Refers to the movement or circulation of air, often in technical or everyday contexts such as room ventilation or airflow in machinery.
I opened the window to improve the ventilation in the room.
This room has poor ventilation, so please open the window.
Please do not block the machine's air vent.
If a closet has poor ventilation, mold tends to grow easily.
密閉 means sealing something airtight so no air can pass, the opposite of 通気, which describes air freely moving through.
通期 (same reading) means 'the whole fiscal period' in business contexts, while 通気 refers to airflow or ventilation.
Compound of 通 (pass through) and 気 (air, spirit), literally 'passing air'. The exact historical derivation is uncertain, but the kanji combination directly conveys the meaning of ventilation.