Irising

Irising is frequently used instead of the fade to indicate the completion of a scene sequence and the beginning of a new series of scenes.

Source: wiktionary

Throughout the film [The Cabinet of Dr. Caligari] circular patterns are symbolic of chaos. Where irising began and ended a happy carnival sequence, an underlying feeling of chaos was created.

Source: wiktionary

The screen is black at first, till a brightly lit hole opens in it and an all-black silhouette stands at what we see, in a moment, is a doorway. This is the wide-screen equivalent of irising in Ford’s silent films.

Source: wiktionary