In the U.S., what are the requirements for being an Executive airport?
This is a marketing distinction -- generally applied to a major GA reliever airport designed to serve light/business jets in design categories B-II and C-II or perhaps C-III.  This is distinguished from general purpose GA relievers as a light GA reliever may be designed to service a maximum design category of A-II, B-I, or B-II; it is also distinguished from County, Municipal, or Regional airports as while they are similarly sized in terms of design category, a County, Municipal, or Regional airport is the primary airport for that area as opposed to being a reliever for a larger scheduled service airport.