definition
strict definition
land boundary
A region boundary clipped to exclude water, ending at the coastline rather than including territorial waters.
A land boundary is the region's outline with the water removed, ending at the coast.
Most large providers publish administrative areas this way, among them the European GISCO and INSPIRE datasets, Natural Earth and the US Census cartographic files. A boundary drawn to include territorial water is the exception rather than the rule.
The difference is not small for a coastal region. Area, population density and any figure derived from area change depending on which of the two is used, so the type has to be recorded with the geometry rather than inferred later.