A black or black and gold magical piece of furniture, very valuable.
The Vanishing Cabinet is a piece of equipment used by Muggle magicians to make people or objects disappear with an illusion. A famous example is the magician known as Don Juan Cardoza (1889-1980). According to Who's Who in Magic History, "(o)ne of his best known tricks was the Vanishing Cabinet, in which he made audience volunteers appear and disappear, and he had a standing '$1000.00 challenge' for anyone who could figure out how he did it."
Ironically, during his first accidental visit to Borgin and Burkes Harry ducked into a Vanishing Cabinet to hide from Draco and Lucius Malfoy; it was very large and black, with double doors (CS4). It had not been sold four years later (HBP6).
Peeves dropped a Vanishing Cabinet in a classroom right over Filch's office, wrecking the Vanishing Cabinet in the process (CS8). Peeves in a sense made Filch "disappear" from his office with this trick because Filch "ran flat-footed from the room."
Fred and George Weasley shoved Montague of the Inquisitorial Squad headfirst into a Vanishing Cabinet on the first floor when he tried to dock points from them on the Inquisitorial Squad's first day. They neither knew nor cared where they had sent him, and figured that it could take weeks for him to get back (OP28).
As it turns out, the Vanishing Cabinet into which Montague was shoved at Hogwarts was malfunctioning - possibly being the same one that Peeves had broken three years before. Montague nearly died while trying to Apparate out (he hadn't yet managed to pass his test) (HBP27).
Thanks to Montague's terrifying experience of being trapped between the two Vanishing Cabinets for an extended period during Draco's fifth year, Draco Malfoy learned that the Vanishing Cabinet at Borgin and Burkes was linked to the damaged Vanishing Cabinet at Hogwarts (HBP27).