The transporter chamber, or transport chamber, was an enclosed space in the transporter room. The transporter platform was located within this chamber. An individual could program a transporter console so that there was a time lag in which the operater could get into the chamber before beaming occurred. (TOS: "Mirror, Mirror"; TNG: "Realm of Fear")

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In the shooting script for Star Trek: The Motion Picture, there was a reference to the transporter chamber.
Though not stated in the canon, the difference between an alcove and a chamber appeared to be dependent on the capacity of the enclosure for holding people.