订飞机票(dìng fēi jī piào) - to book a flight ticket in Mandarin

订飞机票 (dìng fēijīpiào) means to book a flight ticket, built from 订 (dìng), "to book, to order, to reserve", plus 飞机票 (fēijīpiào), "aeroplane ticket" itself made of 飞机 (fēijī), "aeroplane", and 票 (piào), "ticket". Learn it as a chunk and the individual pieces fall into place easily.

订 (dìng) on its own is the general verb for reserving anything ahead of time: 订酒店 (dìng jiǔdiàn), "to book a hotel", 订座位 (dìng zuòwèi), "to reserve a seat". It is worth noting because it looks similar to, but is a different character from, 定 (dìng), "to settle, to fix", which shares the pronunciation but not the meaning.

In real speech, people often shorten the phrase to just 订机票 (dìng jīpiào), dropping 飞 (fēi), since 机票 (jīpiào) already means "plane ticket" on its own. Both the fuller and shorter forms are correct and mean the same thing.