The absolute configuration at a chiral center in a molecule is a time-independent and unambiguous symbolic description of the spatial arrangement of ligands around it.

eg:

The chiral centers in 1 and 2 bear the same ligands: a,b,d, and e.  However, 1 and 2 are not superimposable on each other, meaning that the arrangement of ligands around the chiral center in 1 and in 2 is different.  1 and 2 are mirror images of each other, meaning that the arrangement of ligands around the chiral center in 1 is the exact opposite of that in 2.  Chiral centers in 1 and 2 are said to have opposite absolute configurations.
According to R,S convention, if the absolute configuration at the chiral center in 1 is R, that at the chiral center in 2 is S or vice versa.

See also relative configuration.

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