The term "peers" is commonly used with reference to a group of learners of the same knowledge level and/or discipline. Doctors helping doctors. Writers giving feedback to writers. This gives the impression that peer interaction is limited to where people with similar backgrounds connect.
But what about when doctors and scientists team up to develop new treatments? Or journalists and data crunchers run investigations? Or an engineer and a salesperson start a company?
A person who works on a learning challenge that is similar to another specified person's learning challenge.
In reality, it's not the commonality of the person that matters, but the commonality of the challenge. Different people gather around similar challenges. The challenge defines a peer. It provides the commonality by which learners of various backgrounds connect, not the learners' backgrounds (specific status, age, ability, etc.).
Peer Learning stands out when these challenges are unique, new, or contextual. When there isn't a satisfactory known answer, peer learning allows that answer to be constructed by the learner.