Notation

Description	Example	Meaning
$*j$	$\underline{X}^{*j}$	Denote a matrix with the values as $\underline{X}$ except of $j$-th which elements are permuted
Model residual	$r_i = y_i - f(x_i)$ $r_i = y_i - \hat{y}_i$	The difference between the observed value of the dependent variable $Y$ for the $i$-th observation and the model’s prediction for that observation