Fueki ryūkō (不易流行): the unchanging and the changing. It is a principle of haikai poetry in the school of Matsuo Bashō. A Japanese dictionary explains it this way: the changing fashion that seeks novelty is in fact the unchanging essence of haikai, and the unchanging and the changing must be joined at the root.
Another dictionary notes that no example of Bashō explaining it himself is found. His disciples Kyorai, Dohō and Kyoriku developed it in their writings on haikai.