An Etymological Dictionary of Astronomy and Astrophysics

For any → real number a, the non-negative value of a without regard to its sign; denoted by |a|. Same as → modulus. The absolute value of a is always either → positive or → zero, but never negative. The absolute value of a number may be thought of as its → distance from zero. The following rules hold:
|ab| = |a||b|
|a + b|  ≤  |a| + |b|
|a - b|  ≥  |a| - |b|
For a> 0, |x|  ≤  a if and only if -a  ≤  x  ≤  a.

See also: → absolute; → value.