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We present visual-wavelength photometry and spectroscopy of supernovaSN2008S. Based on the low peak luminosity for a SN of M_R = -13.9 mag,photometric and spectral evolution unlike that of low-luminosity SNe, alate-time decline rate slower than 56Co decay, and slow outflow speeds of600-1000 km/s, we conclude that SN2008S is not a true core-collapse SN and isprobably not an electron-capture SN. Instead, we show that SN2008S more closelyresembles a "SN impostor" event like SN1997bs, analogous to the giant eruptionsof LBVs. Its total radiated energy was 1e47.8 ergs, and it may have ejected0.05-0.2 Msun in the event. We discover an uncanny similarity between thespectrum of SN 2008S and that of the Galactic hypergiant IRC+10420, which isdominated by narrow H-alpha, [Ca II], and Ca II emission lines formed in anopaque wind. We propose a scenario where the vastly super-Eddington wind ofSN2008S partly fails because of reduced opacity due to recombination, assuggested for IRC+10420. The range of initial masses susceptible to eruptiveLBV-like mass loss was known to extend down to 20-25 Msun, but estimates forthe progenitor of SN2008S (and the similar NGC300 transient) may extend thisrange to around 15 Msun. As such, SN2008S may have implications for theprogenitor of SN1987A.

SN 2008S: A Cool Super-Eddington Wind in a Supernova Impostor