What is Fistula?

Legal Definition
A fistula is an abnormal anastomosis, that is, an abnormal connection between two hollow spaces (technically, two epithelialized surfaces), such as blood vessels, intestines, or other hollow organs. Fistulas are usually caused by injury or surgery, but they can also result from an infection or inflammation. Fistulas are generally a disease condition, but they may be surgically created for therapeutic reasons.

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; Eutrochium fistulosum has a tubular stem; and Allium fistulosum has hollow or tubular leaves.
-- Wikipedia
Legal Definition
In the civil law. A pipe for conveying water. Dig. 8, 2, 18.
-- Black's Law Dictionary
Legal Definition
(Civil Law) A conduit.
-- Ballentine's Law Dictionary