Usually what's important about a thesis is not so much the author but the advisor. It would be nice if theses in Zotero could also be searched by this field.
Use the generic "collaborator" role if your purpose is to store and find the advisor. Collaborator can't be cited though, so if you need to display the advisor/supervisor in a citation/bibliography, please provide an example from a a style guide. https://forums.zotero.org/discussion/9136/advisor-as-an-author-type
@Gracile Thanks, that's perfect. Actually I DO NOT want the advisor to appear in the citation (since that is not canonical). I just want to be able to search on it. So "collaborator" seems perfect.
There has been discussion of adding an Advisor role to Thesis (like Contributor, also not mapped to any field/citable), so it may happen, but, as you notice, virtually all of the uses for such a label are realized through using Contributor.
@bwiernik I agree, Contributor seems like a good replacement. However, maybe someday I want to use Contributor for, well, an actual contributor. Then I'll wish I had the advisor field. Why not just add "Advisor"? Is there a counterargument?
No, not really a counter argument. Changes to Zotero fields have been waiting for a long time for Zotero 5.0 to be released (which involved massive changes to the underlying Zotero code). They are now planned for Zotero 5.1. Adviser is one of many changes that is being considered for item types/fields.
Upgrade Storage
https://forums.zotero.org/discussion/9136/advisor-as-an-author-type