hinge line
noun
variants:
or less commonly hinge
Definition of hinge line
1a
: an imaginary line on the earth's surface which can be regarded as a boundary between a stable region and one undergoing upward or downward movement
b
: a line around which one wall of a fault may appear to have rotated with respect to the other wall
2
: the dorsal edge or border of a bivalve shell on which the hinge is situated