This is the 7048th most frequent Chinese word.
登機
"Boarding" (to get on an aircraft).
The term '登機' is used as a noun referring to the act of boarding the airplane.
我正在等待登機。
I am waiting to board the plane.
Here, '登機證' uses '登機' as the noun form for 'boarding', and '證' specifies it as the boarding pass.
請提前準備好登機證。
Please prepare your boarding pass in advance.
In this sentence, '登機' is used as a verb phrase meaning 'to board the airplane'.