I disagree, many people would use 'in' rather than 'at', myself included; similarly, announcements at UK airports use 'in', e.g. 'Please wait in the departure lounge'. I would not dismiss 'at' as a possibility - clearly the other two answerers use it, I'm guessing they are not the only ones; I've just never encountered it myself. Maybe I should clock up some more airmiles and hunt it down.. An airport terminal consists of the terminal building itself, plus associated exterior structures: the aircraft stands (i.e. the flat area for parking aircraft), airbridges, etc. For example on arrival at Gatwick North Terminal, cabin crew used to advise passengers to keep mobiles switched off until they were inside.the terminal building: it is possible to be at the terminal without being inside the terminal building.

We'll meet at the departure lounge.** Terminal, as in a boarding terminal? There are terminals at airports and train-stations - it's the location at which you board the transportation. This is probably what you mean by "departure lounge". If this is the case, you can say: We'll meet at the terminal.

Since terminal is an enclosed place you can use in, but if you like to refer to the exact address as terminal you must use at.

I agree. you definitely have to use the preposition AT