[According to Martin Fowler](http://www.refactoring.com/), code refactoring is (emphasis mine):

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> Refactoring is a disciplined technique for restructuring an existing body of code, altering its internal structure **without changing its external behavior**. Its heart is a series of small behavior preserving transformations. Each transformation (called a 'refactoring') does little, but a sequence of transformations can produce a significant restructuring. Since each refactoring is small, it's less likely to go wrong. The system is also kept fully working after each small refactoring, reducing the chances that a system can get seriously broken during the restructuring.
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What is "external behaviour" in this context? For example, if I apply [move method refactoring](http://www.refactoring.com/catalog/moveMethod.html) and move some method to other class, it looks like I change external behaviour, doesn't it?

So, I'm interested in figuring out at what point does a change stop being a refactor and becomes something more. The term "refactoring" may be misused for larger changes: is there a different word for it?

**Update.** A lot of interesting answers about interface, but wouldn't move method refactoring change the interface?