1. Rotating device which is used to indicate direction of wind. Also called wind or weather vanes. 2. Flat surface that rotates pushing against air or water. 3. Fins that stabilize a towed or dropped object. The otter boards that hold a otter trawl open are vanes. 4. The sighting devices on a backstaff or other nautical instrument based on the derivation of vanes as pieces that stick out at 90 degrees from an axis.