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.
I wasn't expecting to be out this late, so I didn't have my lights with me, and I was a bike ninja for the ride home.

When I crossed the intersection, I was almost hit by a bike ninja! Dude needs to get some lights and look where he's going.

by aprilstarchild May 23, 2008
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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).
"Some bikers wear all spandex, and others wear all black and look like Bike Ninjas."
by lone sentry January 8, 2012
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A bike cop, that hides in bushes and alley-ways looking for completely innocent students, and gives them MIP's etc.
Did you see that bike ninja tackle Eric a while ago.

Yea that ninja needs a stick poked through his spokes.
by C MART October 5, 2005
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An idiotic and naive reference to any motorcycle that resembles the famous line of motorcycles from Kawasaki. More accurately termed sportbike. Sometimes called crotch rocket as well. Often inaccurately lumped together as a group of any non-cruiser motorcycles.
Dude:Hey baby you wanna go for a ride on my motorcycle
Chick: That depends, Is it a Harley or a ninja bike?
by Rob February 27, 2006
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