spring line
Prevents fore-aft movement
A spring line is a mooring rope that runs at an angle from a boat to the dock, preventing the vessel from moving forward or backward while allowing it to rise and fall with tides or waves.
Spring lines typically run from the bow aft or from the stern forward: Secure the spring lines before you leave the boat.
The term comes from the rope's function—it lets the boat "spring" or give with water movement rather than strain against a fixed point.