A boarding gate (at an airport) or an aircraft door; the place where passengers board a plane.
案内表示を見ると、搭乗口は25番だとわかった。
Looking at the signs, I saw the boarding gate was number 25.
搭乗口前で列ができているので、早めに行ったほうがいい。
There's a line forming at the boarding gate, so it's better to go early.
搭乗口が変更になったので、放送でアナウンスがあった。
The boarding gate was changed, so there was an announcement over the PA.