From Sudopedia, the free Sudoku reference guide
Orbit
From Sudopedia
An orbit is a group of cells that must either all contain a given or none of them can.
This term is used to describe the effects of symmetry in a Sudoku.
Orbits can have a size of 1, 2, 4 or 8 for standard symmetry classes, and occasionaly 3, 6 or 9 for alternative symmetry classes. Large orbits increase the chance of redundancy in the givens.
