Where to find it:

NodeGraph / Right Mouse Click / Add Nodes / Math / Extension Pack / A /

The signs of y and x are used to determine the quadrant that the angle lies in. 

The value returned by atan in this case is in the range [−π,π]. The result is undefined if x=0.

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/atan.xhtml

•Y

Specify the numerator of the fraction whose arctangent to return.

•X

Specify the denominator of the fraction whose arctangent to return.

•Output RGB

Separate computation for Input Channels Red, Green, Blue with an Alpha Channel of 1.0

•R

Computation for Input Red channel only, saved into R, G and B channel with an Alpha of 1.0

•G

Computation for Input Green channel only, saved into R, G and B channel with an Alpha of 1.0

•B

Computation for Input Blue channel only, saved into R, G and B channel with an Alpha of 1.0

•A

Computation for Input Alpha channel only, saved into R, G and B channel with an Alpha of 1.0