Absolute Value (Wolfram MathWorld)

The absolute value of a real number x is denoted |x| and defined as the "unsigned" portion of x,|x|=xsgn(x)(1)={-x for x<=0; x for x>=0,(2) where sgn(x) is the sign function. The absolute value is therefore always greater than or equal to 0. The absolute value of x for real x is plotted above. The absolute value of a complex number z=x+iy, also called the complex modulus, is defined as |z|

Abs (Mathematica)

http://reference.wolfram.com/mathematica/ref/Abs.html