A character in a major franchise created by a fan of something, and self inserted into that franchise usually via art and fanfics.
Usually due to the fact that the designs end up being so close to the source media, they end up looking almost like the original, and a ton of OCs are just traces/recolors of offical artwork for the source franchise.
Many of them also happen to be Mary Sues with self insert fanfics and copy/paste backstories.
The creators of them are also known for being very egotistical about them, going as far as putting copyrights on them, telling others not to steal their (possibly stolen anyway) character, and even abusing the report systems at various sites just to be sure nobody will steal their OC.
They exist for almost any franchise, they can exist in everything from Sonic to Pokemon to Naruto to even South Park!
With some franchises, such as Sonic or MLP, there are even OC generators that people will use to generate OCs and put them up on sites like DeviantArt as "art".
Blonic is my original character, DO NOT STEAL!