Other forms: objets d'art
An objet d'art is a piece of art, especially a small work that's three-dimensional, rather than a painting or a large sculpture. Your grandmother's favorite objet d'art might be a small ceramic dish you made in elementary school. Awww.
In French, object d'art simply means "art object", and it really can be used in that literal way to refer to any work of art. It's more common to call small decorative or collectible pieces objets d'art, especially things that truly have some artistic value (rather than your collection of Pez candy dispensers or Star Wars action figures). Often this term is shortened to objet.