In geometry, curvilinear coordinates are a coordinate system for Euclidean space in which the coordinate lines may be curved. These coordinates may be derived from a set of Cartesian coordinates by using a transformation that is locally invertible (a one-to-one map) at each point. 

https://en.wikipedia.org/wiki/Curvilinear_coordinates#:~:text=In%20geometry%2C%20curvilinear%20coordinates%20are,one%20map)%20at%20each%20point.