Fungi or Mushrooms that have certain active components in them which makes them bridge the grey area between 'food' and 'supplement'; can be consumed as food or taken as a loose powder in capsules.
Bioactive Mushrooms (according to Examine.com) is a category for Mushrooms or Fungi that have properties beyond just being delicious or used as foods. These are the mushrooms used traditionally to cure diseases, treat symptoms, or are otherwise called 'Medicinal Mushrooms'.
Most medicinal mushrooms tend to have a beta-glucan (carbohydrate) component that interacts with the immune system, and other compounds such as triterpenoids or flavonoids that can do other unique things.
The beta-glucan component and other fibrous components are known to made a cell wall to the mushroom which may not be digestible to some, which has risen to medicinal mushrooms being marketed as dietary supplements (due to the cell wall being broken when in powder form).