Partial derivative
The partial derivative of a function f of a number of variables, is the derivative in which just one of the variables is treated and the others as constants
in which the symbol ∂ is used.
Explanation
Other notations are
These different notations are often used interchangeably. That seems confusing, but in practice it is quite usefull.
Example 1
The most general form of the Schrödinger equation is
where is the differential operator.
Example 2
The laplacian of a function f in ℝn coordinates is
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where 
is the differential operator.