Pronunciation: (tīd'märk"), [key]
1. the point that something or someone has reached, receded below, or risen above: He has reached the tidemark of his prosperity.
2. a mark left by the highest or lowest point of a tide.
3. a mark made to indicate the highest or lowest point of a tide.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.