border state

1 sometimes capitalized B&S a :  a state (such as Delaware, Maryland, Virginia, Kentucky, or Missouri) bordering on an antislavery state and favoring slavery before the Civil War b :  a state (such as Maryland, West Virginia, Kentucky, Missouri, Oklahoma, or Tennessee) just north of the Solid South and traditionally voting Democratic c :  a state (such as Montana or North Dakota) bordering on Canada

2 :  a small country (such as Poland) bordering on a larger more powerful country; especially :  such a country lying between two larger more powerful countries :  buffer state