What is s-score?

The s-score of a stock is the distance of its cumulative residual from the level it reverts to, in units of its equilibrium standard deviation, $s = (X - m)/\sigma_{\mathrm{eq}}$, from an AR(1) fit of the cumulative residual over a trailing window. Because the regression’s residuals sum to zero, the last point is $X_{60} = 0$ and $s = -m/\sigma_{\mathrm{eq}}$; Avellaneda and Lee centred $m$ across stocks.