Add-on is the term used in the Orbiter community to describe those additional ships, bases and functions that are not included in the basic distribution of Orbiter. An alternative term for such extensions is modification (or mod). These are written by members of the commmunity and freely distributed.
The possible main add-on classes for Orbiter are:
- Solar systems
- Celestial bodies
- Surface bases
- Custom functions
- Plug-ins, which are neither MFD nor custom function
- Launch pad extensions