undermount
/ˌʌndərˈmaʊnt/Definitions
1. noun
a kitchen sink that is built into a countertop, with the rim of the sink below the level of the countertop
“The new kitchen features an undermount sink that adds a sleek touch to the space.”
2. verb
to install a sink below the level of a countertop
“The plumber will undermount the sink in the new kitchen.”