Let's say you got really excited when fiber optic communication became available and decided to lay extensive lines throughout your company.
Before long, however, you realize that you only need a few and wonder what to do with the rest.
The unused optical fibers are what are called dark fiber.
Dark, because the fibers are unlit, unlike optical fibers that send light pulses while transmitting information.
Strands of dark fiber can be sold or leased to individuals or companies wishing to set up fiber-optic communication channels.