  We present a visibility based estimator namely, the Tapered Gridded Estimator
(TGE) to estimate the power spectrum of the diffuse sky signal. The TGE has
three novel features. First, the estimator uses gridded visibilities to
estimate the power spectrum which is computationally much faster than
individually correlating the visibilities. Second, a positive noise bias is
removed by subtracting the auto-correlation of the visibilities which is
responsible for the noise bias. Third, the estimator allows us to taper the
field of view so as to suppress the contribution from the sources in the outer
regions and the sidelobes of the telescope's primary beam. We first consider
the two dimensional (2D) TGE to estimate the angular power spectrum $C_{\ell}$.
We have also extended the TGE to estimate the three dimensional (3D) power
spectrum $P({\bf k})$ of the cosmological 21-cm signal. Analytic formulas are
presented for predicting the variance of the binned power spectrum. Both the
estimators and their variance predictions are validated using simulations of
$150 \, {\rm MHz}$ GMRT observations. We have applied the 2D TGE to estimate
$C_{\ell}$ using visibility data for two of the fields observed by TIFR GMRT
Sky Survey (TGSS). We find that the sky signal, after subtracting the point
sources, is likely dominated by the diffuse Galactic synchrotron radiation
across the angular multipole range $240 \le \ell \lesssim 500$.
