What is a port sidelight on a boat?

A port sidelight on a boat is a navigation light located on the left side (port side) of the vessel when facing forward. It emits a red light and is used to indicate the boat's position and direction to other vessels at night or in low visibility conditions. This light is part of a boat's overall lighting system, which also includes a starboard sidelight (green) and a stern light, helping to ensure safe navigation and prevent collisions.