A term 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.