Stern

The stern is the aft end of the boat. The opposite of the stern is the bow.

Astern means in the direct of the stern. The opposite of astern is forward.

Astern is mostly synonymous with the word aft, although aft usually refers to things inside the boat, and astern usually refers to things outside the boat, or the direction the boat is moving.

Compare:

 Move the cargo aft to create more space in the forward end of the hold.

with

 Before putting the engine in reverse, check astern to make sure there are no lines in the water.