the waters of a littoral state that are regarded as under the jurisdiction of the state: traditionally those waters within three miles (4.8 km) of the shore, but in the 20th century claims by coastal nations have extended to 12 or even 200 miles (19.3 or 321.8 km).
Compare high sea(def 1).
Origin of territorial waters
First recorded in 1870–75
Also called marine belt.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
the waters over which a nation exercises jurisdiction and control
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012