Definition from Wiktionary, the free dictionary
- (nautical) Beyond the stern when viewed from aboard.
- He signalled the engineroom to go astern: in other words, to run the engine in such a way as to put the ship beyond the present position of her stern, as viewed from aboard.
Derived terms 
astern (not comparable)
- (nautical) behind
- If one ship is following another, the first is astern when viewed from the second.
Usage notes 
- Within the ship, the corresponding adjective is abaft. An object nearer the stern than the mainmast is abaft the mainmast.