Packet ships, packet liners, or simply packets, were sailing ships in the early 1800s which departed port on a regular schedule. The typical packet sailed between American and British ports, and the ships themselves were designed for the North Atlantic, where storms and rough seas were common.
The first of the packet lines was the Black Ball Line, which began sailing between New York City and Liverpool in 1818. The line originally had four ships, and it advertised that one of its ships would leave New York on the first of each month. The regularity of the schedule was an innovation at the time.
Within a few years several other companies followed the example of the Black Ball Line, and the North Atlantic was being crossed by ships that regularly battled the elements while remaining close to schedule.
The packets, unlike the later and more glamorous clippers, were not designed for speed. They carried cargo and passengers, and for several decades packets were the most efficient way to cross the Atlantic.
The use of the word "packet" to denote a ship began as early as the 16th century, when mail referred to as "the packette" was carried on ships between England and Ireland.
The sail packets were eventually replaced by steamships, and the phrase "steam packet" became common in the mid-1800s.