tidemark

noun

tide·​mark ˈtīd-ˌmärk 

1

a

: a high-water or sometimes low-water mark left by tidal water or a flood

b

: a mark placed to indicate this point

2

: the point to which something has attained or below which it has receded

the tidemark of tolerance has risen

—New Republic

Word History

First Known Use

1799, in the meaning defined at sense 1a