1) Line on oceanographic maps 
a contour line linking regions of the same depth.
line connecting points of equal underwater depth
A contour of equal depth in a body of water, represented on a bathymetric chart. 
a contour line linking regions of the same depth.
isobath 1. Having the same depth; of equal depth. 2. A line (either imaginary or on a map) joining places where water has equal depth; an underwater (bathymetric) contour.
Line connecting points of equal water DEPTH on a chart; a seabed contour. 
an imaginary line or one drawn on a map connecting all points of equal depth below the surface of a body of water. · a similar line for indicating the depth below the surface of the earth of an aquifer or the top of the water table.