• ship-of-the-line

Contraction ofline of battle ship, a warship large enough to be in theline of battle. See alsobattleship, contracted from the same phrase.

shipoftheline (pluralships of the line)

1. (nautical,military) Alargesquare-riggedwarship large enough to have a place in theline of battle, with up to 140guns on at least twodecks. Acapital ship from theage ofsail, superior to afrigate; usually, aseventy-four, orthree-decker.

• The OED records the termsline of battle ship, battleship,ship of the line andliner being used interchangeably up through the 19th century. In modern usageship of the line is a wooden line of battle ship from the age of sail;battleship is an armoured and motorized warship of the late 19th to mid 20th centuries.

• (square-rigged warship):man-of-war
• (vessel superior to frigate):line of battle ship;liner

•  ship of the line on Wikipedia.Wikipedia

• “line”, inWebster’s Revised Unabridged Dictionary, Springfield, Mass.:G. & C. Merriam,1913,→OCLC.