A Badnik is a term for an enemy robot in the early North American and European versions of the Sonic the Hedgehog video games.
Most badniks are innocent animals that the evil Dr. Robotnik (Dr. Eggman in newer games) has robotized and made evil. Usually, after Sonic destroys the robots, they return into the normal animals that they used to be.
The badnik term fell out of use after Sonic Adventure was released.
Badniks in the UK Comic
In the UK (Edgmont Fleetway) Sonic the Comic all early Badniks were powered by Organic Batteries, the name given to the helpless Mobian trapped inside, however stronger self-sufficient badniks known as Prime Badniks which could operate without the need of such batteries soon appeared. The first such was the Arachbot a spider like badnik which captured Tails in an early episode and was resistant to sonic's attacks until he torched it using the weapon from another badnik he had destroyed. Most badniks continued to use organic batteries with the exception of Troopers and some special project badniks such as the Metallixs. In earlier episodes Dr. Robotnik used mobile factories, which were basically large badniks that injested a Mobian and then Roboticised it releasing it as a badnik however in later episodes Robotnik used huge processing plants to churn out badniks in large numbers.
The most powerful badnik type was the Cybernik, of which only 2 were ever made, Shortfuse and Vermin. Shortfuse was the first, a prototype made of Megatal, an ultra hard metal, designed by Grimer to destroy Sonic the Hedgehog, but it's organic battery Shorty the squirrel took control and joined the fight against Robotnik, unable to escape from his metal shell. Vermin was sent to destroy him but was eventually destroyed by Shortfuse.
All the Badniks on Mobius were immobilised in episode 100 along with all the electrical systems on Mobius including Robotnik's Organic Computer on the Floating Island (but not the Floating Island itself or its ancient Guardian Robots) when Super Sonic burst out of the Black Asteroid in Mobius' orbit, transported from the Special Zone by the Omni Viewer, causing an Electro-magnetic pulse which covered the planet. Robotnik created a new batch from his new base on Flickies' Island but never regained control over the planet again.
A one off Sonic story considered a bit of a fan favourite was that of 'Smokey and the Badnik' in which a Roller badnik (which features in Sonic 1, Springyard Zone) befriends a young Mobian called Smokey, and protects him from a Trooper attack. Sonic spares the Robot once he sees it is harmless and the two walk off into the sunset.