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We report absolutely calibrated measurements of diffuse radio emissionbetween 90 and 190 MHz from the Experiment to Detect the Global EoR Signature(EDGES). EDGES employs a wide beam zenith-pointing dipole antenna centred on adeclination of -26.7$^\circ$. We measure the sky brightness temperature as afunction of frequency averaged over the EDGES beam from 211 nights of dataacquired from July 2015 to March 2016. We derive the spectral index, $\beta$,as a function of local sidereal time (LST) and find -2.60 > $\beta$ > -2.62$\pm$0.02 between 0 and 12 h LST. When the Galactic Centre is in the sky, thespectral index flattens, reaching $\beta$ = -2.50 $\pm$0.02 at 17.7 h. TheEDGES instrument is shown to be very stable throughout the observations withnight-to-night reproducibility of $\sigma_{\beta}$ < 0.003. Includingsystematic uncertainty, the overall uncertainty of $\beta$ is 0.02 across allLST bins. These results improve on the earlier findings of Rogers & Bowman(2008) by reducing the spectral index uncertainty from 0.10 to 0.02 whileconsidering more extensive sources of errors. We compare our measurements withspectral index simulations derived from the Global Sky Model (GSM) of deOliveira-Costa et al. (2008) and with fits between the Guzm\'an et al. (2011)45 MHz and Haslam et al. (1982) 408 MHz maps. We find good agreement at thetransit of the Galactic Centre. Away from transit, the GSM tends toover-predict (GSM less negative) by 0.05 < $\Delta_{\beta} =\beta_{\text{GSM}}-\beta_{\text{EDGES}}$ < 0.12, while the 45-408 MHz fits tendto over-predict by $\Delta_{\beta}$ < 0.05.

Improved Measurement of the Spectral Index of the Diffuse Radio Background Between 90 and 190 MHz