Definitions
1.adverbIn the middle of a ship, either longitudinally or laterally."The main cabin, most amidships, was 8 feet 3 inches broad [...]"
2.adverbUsually in the line of the keel, but sometimes halfway between bow and stern; often contracted to “midships.” (FM 55-501).
3.adverbOn the flank, at a vulnerable place.
4.interjectionA helm order, normally shortened to midships!, to centre the helm in the line of the keel.