A bouillon cube (US) or stock cube (UK) is dehydrated broth (bouillon in French) or stock formed into a small cube (about 15 mm³).
It is made by dehydrating vegetables, meat stock, a small portion of solid fat (such as hydrogenated oil), salt (usually well over 50%) and seasonings (usually including monosodium glutamate) and shaping them into a small cube.
Dehydrated broth is also available in granular form. Broth made from rehydrated cubes is different in taste to fresh broth because of its higher salt content and flavors changed by the boiling process.
Bouillon cubes are convenient and cheap.
The cubes are widely used in British cooking to add flavor.