interliner

noun

1. A protective layer between an external cover and a filling, especially as a fire barrier.
2. A passenger whose journey involves changing from one airline to another.

   “Whilst one quarter of Heathrow's traffic in 2001 was composed of interliners, at Manchester less than two and a half per cent of passengers were changing planes the airport^([sic]). Yet both airports serve around 170 destinations worldwide […]”