HWM

Noun

high-water mark (plural high-water marks)

1. (literal) A mark, such as a line of seaweed, showing the highest level reached by a body of water.
   • This imposing structure, which not only carries the single track branch line but also the gas main supplying the Forest of Dean area, cost £200,000 when it was built in 1879, and at its highest point is 70ft above the high-water mark.1961 January, “Talking of Trains: The Severn Bridge disaster”, in Trains Illustrated, page 5

   Antonyms:

   low-water mark

2. (broadly, figuratively) The peak, apex or acme of something; the maximum level; the furthest or highest point.
   • The Gettysburg campaign was the high-water mark of the Confederacy's invasions of the North.

   Antonyms:

   low-water mark