sideboard vs piece of furniture

sideboard and piece of furniture both are nouns.

Word	Noun	Adjective	Verb	Adverb
sideboard	Yes	No	No	No
piece of furniture	Yes	No	No	No

As nouns, piece of furniture is a hypernym of sideboard; that is, piece of furniture is a word with a broader meaning than sideboard:

sideboard: a piece of furniture that stands at the side of a dining room; has shelves and drawers
piece of furniture: furnishings that make a room or other area ready for occupancy

Other hypernyms of sideboard include article of furniture, furniture.

sideboard (noun)	piece of furniture (noun)
a piece of furniture that stands at the side of a dining room; has shelves and drawers	furnishings that make a room or other area ready for occupancy
a board that forms part of the side of a bed or crib
a removable board fitted on the side of a wagon to increase its capacity