# Definition of curvilinear coordinates?

Please can someone give me a formal definition of curvilinear coordinates, preferably with as source. The once that I have found don't seem to be very formal.

• What's wrong with the wiki pages on this topic? It looks pretty extensive. – Gregory Grant May 24 '15 at 13:30
• @GregoryGrant They don't give a formal definition, in the first line it says ' A coordinate system,..., where the coordinate lines may be curved.' But this is not very formal, or even a definition, I can't clear definition else where on the page. – Quantum spaghettification May 24 '15 at 13:34