"The Weighing of the Wands is about to start, and it cannot take place if one of our champions is hidden in a broom cupboard."
-- Albus Dumbledore to Rita Skeeter (GF18)
A Broom Cupboard is a small room for storing brooms and other supplies.
Rita Skeeter pulls Harry Potter into a broom cupboard in the small room next to the Great Hall after he is selected for the Triwizard Tournament and then proceeds to conduct a sneaky interview with him for the Daily Prophet (GF18, GF19).
The Room of Requirement once turned itself into a broom cupboard so that Fred and George Weasley could hide from Mr Filch in there (OP18).
In answer to Hermione Granger's question during their History of Magic lesson, Professor Binns denies that there is any evidence of Salazar Slytherin having built a broom cupboard inside Hogwarts, much less a concealed secret chamber (CS9).
Mr Weasley's office in the Ministry of Magic, the Misuse of Muggle Artefacts, is located next to a broom cupboard and compares unfavourably to it in size (OP7).