An international port or an area within an international port at which, crew, passengers, baggage, cargo, mail and stores may be disembarked or unloaded, may remain and may be transhipped, without being subjected to any customs charges or duties. (Examination is possible for instance to meet security or narcotics control requirements.) Source: IMO.
A free port (porto franco) or free zone (US: Foreign-Trade Zone) is a port or area with relaxed jurisdiction with respect to the country of location. Most commonly this means being free of customs or being a special customs zone with favorable customs regulations. Earlier in time some free ports enjoyed political autonomy.