Ludemes are components that make up the formal abstract element of a game (i.e. the system itself). The topology of a game, such as a "get to the other side" element, would be an example of a ludeme.
This term was first coined by game designer Ben Cousins, and appears in chapters 7 and 10 of Raph Koster's A Theory of Fun for Game Design.
The concept also is very similar to Ian Bogost's unit, and the part it plays in unit operations.