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**What does "Up to Isomorphism" mean?**

I was reading the final chapter in Spivak's Calculus and it says:

There is a complete ordered field and, up to isomorphism, only one complete ordered field.

I know what an isomorphism is and what it means when things are isomorphic. But I don't know what he means when it says "UP to..". I've also read things like this when talking of homotopy...

Any help is appreciated!!