The parent furoxan (1,2,5-oxadiazole 2-oxide), synthesized from glyoxime and NO2(g), has been investigated in the gas phase for the first time by mid-infrared and He I photoelectron spectroscopy, and in the liquid phase by Raman spectroscopy. The ground-state geometry has been obtained from quantum-chemical calculations at the B3LYP, MPn (n = 2-4), CISD, QCISD, CCSD, CCSD(T), RSPTn (n = 2,3), MRCI, and MR-AQCC levels using 6-311++G(2d,2p), cc-pVTZ, aug-cc-pVTZ, cc-pCVTZ, and cc-pVQZ basis sets. Furoxan is predicted to be planar, with a strong exocyclic and a relatively weak endocyclic N-O bond. The furoxan moiety is electron rich, indicated e.g. by a large negative NPA charge (-0.46 e). According to various aromaticity indices, furoxan is nearly as aromatic as furan and furazan. Unlike alkyl- and cyano-substituted furoxans, the parent furoxan, upon thermolysis, does not cleave to the monomer nitrile oxide, yielding only HNCO, HCN, CO2, CO, NO, and H2O decomposition products.