Third-rate was the designation used by the Royal Navy for a ship of the line mounting 64 to 80 guns, typically built with two gun decks.
When the rating system was first established, in the 1670s, the third rate was defined as 70 guns, with second-rates having 90 guns, and fourth-rates 54-60 guns. As time passed, and different ships were built with greater or fewer numbers of guns, the term was expanded to include the whole range from 64 to 80.
This designation became especially common because it included the 74-gun ship, which eventually came to be the most popular size of large ship for navies of several different nations. Although the rating system was only used by the Royal Navy, British authors might still use "third-rate" to speak of a French 74.
By the end of the 18th century, the rating system had mostly fallen out of common use, ships of the line usually being characterized directly by their number of guns, the numbers even being used as the name of the type, as in "a squadron of three 74s".
Third-rate is also used as an adjective to mean that something is of inferior quality.