Real Dictionary

duty-free
adjective: Duty-free goods are sold at airports or on planes or ships at a cheaper price than usual because you do not have to pay import tax on them.

duty-free in American English
designating or of goods on which no duty or tax must be paid, or a shop selling such goods
free of customs duty
with no payment of a duty or tax required

duty-free in British English
with exemption from customs or excise duties
noun: goods sold in a duty-free shop

duty-free in Retail
adjective: Duty-free goods are exempt from the government tax normally charged on that category of goods.