the point that something or someone has reached, receded below, or risen above: He has reached the tidemark of his prosperity.
a mark left by the highest or lowest point of a tide.
a mark made to indicate the highest or lowest point of a tide.

1790–1800; tide1 + mark1

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World English Dictionary
tidemark (ˈtaɪdˌmɑːk)
1.  a mark left by the highest or lowest point of a tide
2.  a marker indicating the highest or lowest point reached by a tide
3.  chiefly (Brit) a mark showing a level reached by a liquid: a tidemark on the bath
4.  informal chiefly (Brit) a dirty mark on the skin, indicating the extent to which someone has washed

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
tidemark   (tīd'märk')  Pronunciation Key 
A line or mark on a shore indicating the highest or lowest level reached by the tide.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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