Definition: Internal borders
(a) the common land borders, including river and lake borders, of the Member States;
(b) the airports of the Member States for internal flights;
(c) sea, river and lake ports of the Member States for regular ferry connections.
European Union, Regulation (EC) No 562/2006 of the European Parliament and of the Council of 15 March 2006 establishing a Community Code on the rules governing the movement of persons across borders (Schengen Borders Code)