Astern means toward or at the back of a ship or boat. When a sailor looks astern, they're looking behind them, toward the stern (the rear end of the vessel). If another boat is following yours, you'd say it's astern of you.

The word works like “backward” does on land, but specifically for watercraft. A captain might order the engines to run astern, meaning in reverse, to slow down or back away from a dock. During naval battles in the age of sailing ships, a commander needed to know which enemy vessels were ahead (in front) and which were astern (behind), since most cannons could only fire from the sides.

You might also hear astern in phrases like “fall astern,” which means to drop behind other ships, similar to falling behind in a race. The opposite of astern is ahead or forward, toward the bow (front) of the ship.