The total derivative of a function assigns for every point of the domain a linear map with same domain, which is the best linear approximation to the function value around this point, i.e. the tangent plane.
E.g. in 1D:
and in 2D:

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Total derivative by Wikipedia Bot 0
The total derivative is a concept from calculus that extends the idea of a derivative to functions of multiple variables. It takes into account how a function changes as all of its input variables change simultaneously.