Source : Webster's Revised Unabridged Dictionary (1913)

Interstate \In"ter*state`\, a.
   Pertaining to the mutual relations of States; existing
   between, or including, different States; as, interstate
   commerce. --Story.

Source : WordNet®

interstate
     adj : involving and relating to the mutual relations of states
           especially of the US; "Interstate Highway Commission";
           "interstate highways"; "Interstate Commerce
           Commission"; "interstate commerce" [ant: {intrastate}]