HMS Britannia, also known as Old Ironsides, was a 100-gun first-rate ship of the line of the Royal Navy.

In the rating system of the Royal Navy used to categorise sailing warships, a first rate was the designation for the largest ships of the line. Originating in the Jacobean era with the designation of Ships Royal capable of carrying at least 400 men, the size and establishment of first-rates evolved over the following 250 years to eventually denote ships of the line carrying at least 80 guns across three gundecks. By the end of the eighteenth century, a first-rate carried no fewer than 100 guns and more than 850 crew, and had a measurement (burthen) tonnage of some 2,000 tons.