A line marking the highest level reached. high water mark

ETYM Old Eng. level, livel, Old Fren. livel, French niveau, from Latin libella level, water level, a plumb level, dim. of libra pound, measure for liquids, balance, water poise, level. Related to Librate, Libella.
1. Establishes the horizontal when a bubble is centered in a tube of liquid; SYN. spirit level.
2. Height above ground.

A line marking the lowest level reached.

For indicating the level of water in e.g. a tank or boiler or reservoir; SYN. water gage, water glass, watergauge, water-gauge.