Professor Melquiades Gamboa:
"High seas ... refers to all parts of the sea not included in the territorial sea or in the internal waters of a state. Being the common property of all nations, no portion of the high seas can be appropriated by any state (but see continental shelf) and no state may subject any part of them to its sovereignty.
"The freedom of the high seas includes: freedom of navigation, freedom of fishing, freedom to lay submarine cables and pipelines, and freedom of overflight."
In 1958, the United Nations proposed a Convention on the High Seas, often referred to as the Geneva Convention on the High Seas. The definition is essentially identical to that proposed by Gamboa above ("... the term high seas means all parts of the sea that are not included in the territorial sea or in the internal waters of a State").
The term was again institutionalized by the United Nations in the 1982 United Nations Convention on the Law of the Sea, also known as UNCLOS.
UNCLOS defines the term by elimination:
"... all parts of the sea that are not included in the exclusive economic zone, in the territorial sea or in the internal waters of a State, or in the archipelagic waters of an archipelagic State."
In essence, the high seas refers to the open ocean not within the territorial waters or jurisdiction of any particular state.
The term is also used in the COLREGS 1972 (collision regulations).
Historically, and prior to the advent of UNCLOS, the common law took the high seas to be all waters past the low tide mark, which also then marked the limit of the jurisdiction of the realm of England. Since then, all states have asserted jurisdiction over a band of water off the coast.
Note also these words of Justice Tjoflat of the United States Court of Appeals in United States v Postal:
"Beyond the territorial sea lie the high seas. These waters are freely accessible to all nations and are not subject to the sovereignty of any nation.
"The regulation of a vessel on the high seas is normally the responsibility of the nation whose flag that vessel flies, and of that nation alone. Ships shall sail under the flag of one State only and, save in exceptional cases expressly provided for in international treaties or in these articles, shall be subject to its exclusive jurisdiction on the high seas."
- Duhaime, Lloyd, United Nations Convention on the Law of the Sea
- Duhaime.org, International Law
- Duhaime.org, Maritime Law
- Gamboa, Melquiades, A Dictionary of International Law and Diplomacy (Quezon City, Philippines: Phoenix Press, 1973), pages 137-138
- United States v Postal, 589 F. 2d 862 (1979)