Absolute value

The absolute value of any number "x" is denoted by |x| such that

|2| = 2 , |0| = 0

and |-4| =-(-4) = 4

It is to be noted that absolute value of a number cannot be negative.

Further Reading:
Cracking the GRE, 2010 Edition (Graduate School Test Preparation)

Kaplan GRE Exam Vocabulary in a Box

Barron's GRE

Essential Words for the GRE (Barron's Essential Words for the GRE)

Kaplan GRE Exam 2010-2011 Premier with CD-ROM (Kaplan GRE Premier Program (W/CD))