The buildings that provide access to the airplanes (via gates) are typically called concourses. However, the terms “terminal” and “concourse” are sometimes used interchangeably, depending on the configuration of the airport.
So it seems the final answer from Wikipedia is that a terminal is the whole thing, and a collection of gates is a concourse, but a concourse is also a terminal, even though the concourse is in the terminal, which means that terminals are in terminals, and the rate at which an airport grows large enough for their concourses to be terminals is called the “terminal velocity”.
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