What is the definition or description of: black hairy tongue?

Black Hairy Tongue: A condition characterised by hyperkeratosis of darkened, hyperplastic filiform papillae on the dorsum of the tongue with secondary hemosiderin deposition. Black hairy tongue is anterior to the circumvallate papillae, due to prolonged antibiotic therapy, resulting in overgrowth of chromogenic bacteria. http://medical-dictionary.thefreedictionary.com/black+hairy+tongue.

Fungal growth.: The tongue has a mixture of bacteria and fungus on it all the time. At times, such as long-term antibiotic usage, the bacterial level drops and leaves room for overgrowth of the fungi. When overgrown, the tongue looks "hairy" due to little stalks of fungus sticking out.

Stained tongue : Sometimes the papillae of the tongue(the pumps on the surface of the tongue) gets elongated, normally they wear away from movements and food chewing. But if they do get elongated , they trap food particles and get stained from food and tobacco and they turn yellow, brown or black the condition looks alarming but actually it is harmless and usually goes away by itself. Brushing the tongue 2/day helps.

Harmless condition: Black hairy tongue is a buildup of too much bacteria and yeast on the tongue. The tiny projections on the tongue called papillae collect the bacteria and instead of shedding the bacteria, the papillae become elongated giving a hairy appearance and black color. Treatment consists of good oral hygiene including gently brushing the tongue and lots of water.

Benign and temporary: Black, hairy tongue is a temporary, harmless (benign), painless oral condition that gives your tongue a dark, furry appearance. The distinct look of black, hairy tongue usually results from an overgrowth of bacteria in the mouth. Although black, hairy tongue may look alarming, it doesn't cause any health problems. Black, hairy tongue usually resolves without medical treatment.

Thrush: This is called monaliasis medically. It is causes by a fungus and often occurs in immune compromised people.