That part of the twilight period during which the horizon may be observed as a sharply defined line and the navigational stars may be seen. The light diffused during this period. For computation purposes, this twilight begins or ends when the middle of the sun is at a point about 10° below the horizon. See civil twilight and nautical twilight.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved