From the Greek isos - equal and bathos - depth.
Isobath - a closed curve on the map passing through points of equal depth. A line on the map connecting points of equal depths of water basins (oceans, seas, lakes, etc.). Often the areas between adjacent isobaths are colored one or another shade of blue, with lighter areas corresponding to shallower depths.
The depth reference point is chosen depending on the type of water body:
For non-tidal seas, zero is the mean sea level;
for tidal seas, zero is the minimum water level (at maximum low tide);
for lakes, rivers and other isolated bodies of water, the reference point is the conditional zero determined by the footstick.