A type of cable that transmits data as light through strands of glass instead of electricity through copper. Fiber-optic cable is a wonderful thing; it can transmit almost insane amounts of data per second, and it is completely impervious to surges, magnetic fields, lightning, and all the other EM nasties that can affect copper cable. Unfortunately it's not impervious to hicks bulldozing big holes in the ground for their swimming pools, but I doubt anything ever will be.

A few layer-1 protocols that are used over fiber include OC-3, OC-12, OC-48, 100base-FL, 100base-FX, and many, many more.