|x| = x if x >= 0, -x if x < 0. Equivalently, |x| = sqrt($x^2$). Geometrically, |x| is the distance of x from the origin, and |x-a| is the distance of x from point a on the number line. This distance interpretation is powerful: |x-2| < 3 means "x is within 3 units of 2," i.e., x in (-1, 5).