What does the word "threesome" mean?

Double flight is a common phrase in tourism, which means that all round-trip traffic is by plane. There is also a saying like solo flight, which means that only one trip is by plane. Among them, similar nouns are "double sleeper" (both are train sleepers) and "single sleeper" (one plane, one train sleeper).

For example, there is a five-day tour from Beijing to Hainan, that is, you fly from Beijing to Haikou, and then fly back to Beijing from Haikou after playing.

A double flight means a round-trip ticket or a double ticket.