What is Filum?

Legal Definition
Lat. In old practice. A file; l. e., a thread or wire on which papers were strung, that being the ancient method of filing. An imaginary thread or line passing through the middle of a stream or road, as in the following phrases:
See also
-- Black's Law Dictionary
Legal Definition
The middle; the thread of anything; as filum aqua; filum viae.
-- Bouviers Law Dictionary
Legal Definition
A thread or edge.
-- Ballentine's Law Dictionary