A landmark visible from the sea, used as a guide in navigation.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "seamark" in a sentence
"Onward they sailed along the south bank of the estuary, past the great sea-carved stone arches of “Île Percée” that made it an important seamark."
"The men came back saying they'd set up a seamark, but there was no place for a port."
"The ruins of the Cistercian Church which once graced this shore and raised above the trees its lighthouse tower, a seamark by day and a beacon by night, are among the loveliest in Wessex."