Variance is also called MS, Mean Square; approximated by the sum of squares (SS) divided by the degrees of freedom (i.e., n-1); the MSE from the empty model can be thought of as roughly the average squared deviation.
Confounding variable is a variable that, though not measured, causes variation in both the explanatory variable and the outcome variable; also know as a "lurking variable" or a "third variable".
Categorical variable is a variable for which the values don’t tell us anything about quantity; the values simply tell us which category the object belongs to.