Collaborations (collabs for short) are levels made by at least 2 persons. When there are way more people that builds a part in the level, (for instance, 10 persons) it is called a mega-collaboration.
Some people even call a level made by an enormous amount of people (more than 30 people for instance) an ultra-collaboration. However, this term is rarely used as there are not many levels that fits this requirement. An example of this is Devastation by Lemons, which is an ultra-collaboration of 46 creators.