# What is a diffeomorphism?

I'm looking for a simple (i.e. as just a rough outline with little differential geometry) definition/ explanation of what a diffeomorphism is. I tried reading the Wiki page but it made no sense to me as a physicist.

To give some context, I'm reading a book which says that because some property of a tensor field $h_{\mu\nu}(x)$ is preserved under infinitesimal transformation of coordinates by the fields $\xi_{\mu}(x)$, then these diffeomorphisms are a symmetry of the theory.

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