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 Space Flight Simulator. 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:
- Custom functions
- Plug-ins, which are neither MFD nor custom function
Vessels (Spacecraft, Launchers, Aircraft)
- List of dwarf planet add-ons
- List of planet add-ons
- List of natural satellite add-ons