a long, narrow ditch

dig a trench around the perimeter of the fire

a long, narrow, deep depression in the ocean bed, typically one running parallel to a plate boundary and marking a subduction zone

the Marianas Trench

a trench coat

dig a trench or trenches in (the ground)

she trenched the terrace to a depth of 6 feet

border closely on; encroach on

this would surely trench very far on the dignity and liberty of citizens