Is resveratrol also a flavonoid?

No. Resveratrol belongs to the nonflavonoid group of polyphenols. Calling it a polyphenol is correct; calling it a flavonoid is not. The terms polyphenol and stilbene both describe resveratrol, but at different levels of classification. Resveratrol has several chemical classifications, because apparently one filing cabinet was not enough.1

What is its chemical structure?

Resveratrol has a stilbene structure with three hydroxy groups attached at positions 3, 5 and 4'. A hydroxy group is an oxygen and hydrogen group. Resveratrol is also classified as a stilbenol and a member of the resorcinols. These are additional chemical descriptions of the same molecule, not alternatives to its classification as a polyphenol.2

Another chemical name for resveratrol is 3,5,4' trihydroxystilbene, which gives the same molecule considerably more paperwork. The numbered positions describe where the three hydroxy groups sit on its structure.12

Is resveratrol a polyphenol or an antioxidant?

It is both. Polyphenol describes resveratrol's chemical classification, while antioxidant describes one of its biological roles. The two terms answer different questions: what kind of compound resveratrol is, and what activity it has.2

Where this answer comes from

  1. 1Resveratrol - MeSHncbi.nlm.nih.gov
  2. 2NIH 3D3d.nih.gov