Oud, oudh, or aoud is a fungus-infected heartwood from the Aquilaria tree (agarwood). This infection changes the tree and the infected area is harvested as oud. This area is called oud wood as it is a piece of wood, later turned into an essential oil.
Once the oud is removed from the tree, it has to be cleared of all the healthy or uninfected parts of the tree. If for some reason this process isn’t very precise, the oud will be spoiled.