In botany, the term is most common in its adjectival forms, where it is used in binomial names to refer to species that are distinguished by hollow or tubular structures. Monarda fistulosa, for example, has tubular flowers. The term was first used in the 14th century. Definition
A fistula is an abnormal connection between vessels or organs that do not usually connect. It can be due to a disease or trauma, or purposely surgically created. Classification
Various types of fistulas include:

Blind: Only one open end; may also be called sinus tracts. Complete: Both internal and external openings. Incomplete: An external skin opening that does not connect to any internal organ.Although most fistulas are in forms of a tube, some can also have multiple branches. Location
Types of fistula can be described by their location. Anal fistulas connect between the epithelialized surface of the anal canal and the perianal skin. Anovaginal or rectovaginal fistulas occur when a hole develops between the anus or rectum and the vagina. Colovaginal fistulas occur between the colon and the vagina. Urinary tract fistulas are abnormal openings within the urinary tract or an abnormal connection between the urinary tract and another organ such as between the bladder and the uterus in a vesicouterine fistula, between the bladder and the vagina in a vesicovaginal fistula, and between the urethra and the vagina in urethrovaginal fistula.
When occurring between two parts of the intestine, it is known as an enteroenteral fistula, between the small intestine and the skin as an enterocutaneous fistula, and between the small intestine and the colon as a colocutaneous fistula.The following list is sorted by the International Statistical Classification of Diseases and Related Health Problems. H: Diseases of the eye, adnexa, ear, and mastoid process
(H04.6) Lacrimal fistula
(H05.81) Carotid cavernous fistula
(H70.1) Mastoid fistula
Craniosinus fistula: between the intracranial space and a paranasal sinus
(H83.1) Labyrinthine fistula
Perilymph fistula: tear between the membranes between the middle and inner ears
Preauricular fistula
Preauricular fistula: usually on the top of the cristae helicis of the ears

I: Diseases of the circulatory system
(I25.4) Coronary arteriovenous fistula, acquired
(I28.0) Arteriovenous fistula of pulmonary vessels
Pulmonary arteriovenous fistula: between an artery and vein of the lungs, resulting in shunting of blood. This results in improperly oxygenated blood. (I67.1) Cerebral arteriovenous fistula, acquired
(I77.0) Arteriovenous fistula, acquired
(I77.2) Fistula of artery

J: Diseases of the respiratory system
(J86.0) Pyothorax with fistula
(J95.0) Tracheoesophageal fistula, between the trachea and the esophagus. This may be congenital or acquired, for example as a complication of a tracheostomy. K: Diseases of the digestive system
(K11.4) Salivary gland fistula
(K31.6) Fistula of stomach and duodenum
(K31.6) Gastrocolic fistula
(K31.6) Gastrojejunocolic fistula – after a Billroth II a fistula forms between the transverse colon and the upper jejunum (which, post Billroth II, is attached to the remainder of the stomach). Fecal matter passes improperly from the colon to the stomach and causes halitosis.