What is Reishi Mushroom or Lingzhi ?

There are several sources defining Lingzhi mushroom, also called reishi:

Linghzi or Reishi Mushroom - Genoderma lucidum

The lingzhi mushroom or reishi mushroom  literally: "supernatural mushroom") encompasses several fungal species of the genus Ganoderma, and most commonly refers to the closely related species, Ganoderma lucidum and Ganoderma tsugae. G. lucidum enjoys special veneration in East Asia, where it has been used as amedicinal mushroom in traditional Chinese medicine for more than 2,000 years, making it one of the oldest mushrooms known to have been used medicinally.  The fungus Ganoderma lucidum. It is also called the reishi mushroom or in Chinese ling zhi. It is one of the most popular medicinal mushrooms in China, Japan, and the United States.Ganoderma grows on logs or tree stumps. It has a shiny, hard, asymmetrical cap that ranges in color from yellow to black. The cap, spores, and mycelium are all used medicinally. Wild ganoderma is rare in Asia.

In ancient China, ganoderma was so rare and so highly prized that it was reserved for the emperors and called the "Elixir of Life." In 1972, Japanese researchers successfully cultivated the mushroom. There are six different colors of cap: red, green, white, black, yellow, and purple. These researchers showed that all colors are the same species, and that the color variations are the result of differences in environmental conditions. Despite this, some herbalists insist that certain colors of reishi mushroom are more potent or effective in healing certain conditions than others.

Lingzhi is a polypore mushroom that is soft (when fresh), corky, and flat, with a conspicuous red-varnished, kidney-shaped cap and, depending on specimen age, white to dull brown pores underneath. It lacks gills on its underside and releases its spores through fine pores, leading to its morphological classification as a polypore.