Mushrooms that have been sliced and dried as a preservation method. Most dried mushrooms can be reconstituted in warm water for about 30 minutes, then squeezed out and used in a recipe calling for mushrooms.
Typically if you can find a type of fresh mushrooms you can also find its dried counterpart with one main exception, the common white button mushroom. Common dried mushrooms include porcini, shiitake, morels, and chanterelles.