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"A ship of war large enough to have a place in the line of battle."
This definition needs expansion. The ship of the battleline sailed nobly for centuries as the backbone of fleets. It was the mighty mobile fortress that could hit the hardest blows and take the most punishment-for in war one must expect to suffer as well as to harm.
In the days of sail the magnificent man of war that made up the battleline came to be called Ships-of-the-Line-matchless in grace, speed, and beauty. When iron and steam replaced wood and sail, the successor that evolved for the sailing ship of the battleline at the turn of the century came to be called battleship.
http://www.hazegray.org/danfs/line/sotl.htm#inde