In naval terminology, a destroyer is a fast, manoeuvrable, long-endurance warship intended to escort larger vessels in a fleet, convoy, or battle group and defend them against powerful short-range attackers. They were originally developed in 1885 by Fernando Villaamil for the Spanish Navy as a defense against torpedo boats, and by the time of the Russo-Japanese War in 1904, these "torpedo boat destroyers" (TBDs) were "large, swift, and powerfully armed torpedo boats designed to destroy other torpedo boats". Although the term "destroyer" had been used interchangeably with "TBD" and "torpedo boat destroyer" by navies since 1892, the term "torpedo boat destroyer" had been generally shortened to simply "destroyer" by nearly all navies by the First World War.

Before World War II, destroyers were light vessels with little endurance for unattended ocean operations; typically, a number of destroyers and a single destroyer tender operated together. After the war, destroyers grew in size. The American Allen M. Sumner-class destroyers had a displacement of 2,200 tons, while the Arleigh Burke class has a displacement of up to 9,600 tons, a difference of nearly 340%. Moreover, the advent of guided missiles allowed destroyers to take on the surface-combatant roles previously filled by battleships and cruisers. This resulted in larger and more powerful guided-missile destroyers more capable of independent operation.

At the start of the 21st century, destroyers are the global standard for surface-combatant ships, with only two nations (the United States and Russia) officially operating the heavier cruisers, with no battleships or true battlecruisers remaining. Modern guided-missile destroyers are equivalent in tonnage but vastly superior in firepower to cruisers of the World War II era, and are capable of carrying nuclear-tipped cruise missiles. At 510 feet (160 m) long, a displacement of 9,200 tons, and with an armament of more than 90 missiles, guided-missile destroyers such as the Arleigh Burke class are actually larger and more heavily armed than most previous ships classified as guided-missile cruisers. The Chinese Type 055 destroyer has been described as a cruiser in some US Navy reports due to its size and armament.

Some NATO navies, such as the Canadian, French, Spanish, Dutch, and German, use the term "frigate" for their destroyers, which leads to some confusion.