buoy
a distinctively shaped and coloured float, anchored to the bottom, for designating moorings, navigable channels, or obstructions in a body of water
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
buoy
[bȯi] (engineering)
An anchored or moored floating object, other than a lightship, intended as an aid to navigation, to attach or suspend measuring instruments, or to mark the position of something beneath the water.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.