Someone who is riding their bicycle in dark/low visibility conditions, without a headlight or taillight. So-called because they are invisible, like a ninja.

The webcomic Yehuda Moon had a comic illustrating a bike ninja on April 14th.

Bike Ninjas are people who wear all black, and and become very ninja-looking when wearing a face mask / balaclava in cold weather. The all black also adds to their "invisibility" while riding at night (which can be dangerous). Bike Ninjas are often urban dwellers, ride fixed-gear or single-speed bikes, and are usually "hipsters" of some sort (hence the all black clothes).

A bike cop, that hides in bushes and alley-ways looking for completely innocent students, and gives them MIP's etc.