I've heard the term edge to edge routing several times, but I'm having a hard time finding a formal definition as to what exactly this means. Can someone please explain what edge to edge routing is?
Like most IT terms, there is no formal definition. It means whatever the vendor wants it to mean.
But edge-to-edge routing in a cloud provider context means essentially using a "virtual cloud"** as a transit between another cloud and some other network (i.e., routing from one edge of the cloud to another).
For example, you have a corporate network connected to one Amazon VPC (A). That VPC is also connected to another VPC (B). Routing from your corporate network through VPC A to reach VPC B is an example of edge to edge routing (and it's not allowed).
Here is an Amazon document that uses the term extensively.
** Cloud providers have different product names for this: Virtual Private Cloud or Virtual Network.