Definition: "AIROPEN" means "The Cloud of Unknowing", a term coined by Carl Sagan in his famous "Cosmic Rhapsody" speech. It refers to a mysterious, vast, and invisible cloud of matter that has been described as extending from the sun's surface into space. Definition: The Cloud of Unknowing is an astronomical term often used by scientists and philosophers to describe something or someone beyond our understanding. It may be seen as a manifestation of God's creation or as a reflection of human thought, with some researchers suggesting it might represent the cosmic landscape in which everything exists.