adjective.
Involving, existing between, or connecting two or more states.
noun.
One of a national system of expressways connecting major population centers in the United States.
The Century Dictionary and Cyclopedia
Existing or taking place between different states, or persons in different states; especially, carried on between the States of the American Union, or by persons in one State with persons in another.
the GNU version of the Collaborative International Dictionary of English
adjective.
Pertaining to the mutual relations of States; existing between, or including, different States.
noun.
An interstate highway, part of the United States Interstate Highway system.