How should I list someone as reference? I am writing an application where I should indicate my relationship with those that provide references for me. How should I describe the following: the person has never been my supervisor, however as he knows my PI very well, he is familiar with my work and we also had several lengthy informal discussions. How could I summarize that on an application?

Thank you for any help!
Something like Colleague might work. Something like "informal advisor". It depends a bit on how much room you have to detail it, as well as the closeness of the relationship. If the person has a good, independent, reputation, then something like that should do fine. At some future point you might be asked to expand on it.