A bay is a well-marked indentation whose penetration is in such proportion to the width of its mouth as to contain land-locked waters and constitute more than a mere curvature of the coast. An indentation shall not, however, be regarded as a bay unless its area is as large as, or larger than, that of the semi-circle whose diameter is a line drawn across the mouth of that indentation.
United Nations. 1982. United Nations Convention on the Law of the Sea. PART II. Territorial sea and contiguous zone. Article 10. https://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf