How to describe the relationship with writer of recommendation letter? I am applying for tenure track /postdoc and I asked two professor to write me a research letter. In some job applications I am asked to describe the relationship with them, and I am not so sure what are the correct words. 

The first one is a professor in the university where I obtain the PhD. They are also an expert in the field. 

The second one is a world-class expert in my field. I know them personally as we meet in conference over the years and I visited them occasionally.

I did not collaborate with both of them. 

Any suggestions?
Both don't seem like unusual "relationships" at all, so I don't think there should be any issue describing them just like you did here.

If there is a drop-down list, you can select whatever is the most appropriate (presumably there are options amounting to "colleague at my current department" and "collaborator" - I think it counts as "collaborating" if you have visited them occasionally, even if you happened to not have published a paper yet). If there is a freetext field, you can just write what you wrote here.