Where to find it:

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

distance returns the distance between the two points p0 and p1. i.e., length(p0 - p1);
Component Selection can be done in the Node's properties.

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

•Point 1

Specifies the first of two points

•Point 2

Specifies the second of two points

•Output RGB

Returns the distance between two points