From Longman Dictionary of Contemporary Englishcode-sharingˈcode-ˌsharing noun [uncountable] when two airline companies sell tickets together and use the same numbers for their flights
Examples from the Corpus
code-sharing• Countless travelers get similar surprises when flying both in the United States and abroad because of a mushrooming arrangement called code-sharing.From Longman Business Dictionarycode-sharingˈcode-ˌsharing noun [uncountable]COMMERCE when two AIRLINEs sell tickets together and use the same numbers for their flightsThe two airlines are expected to announce a strategic partnership which might include a code-sharing agreement. 