Meaning: 

1.  a thing belonging or appropriate to a period other than that in which it exists, especially a thing that is conspicuously old-fashioned.
2. the action of attributing something to a period to which it does not belong.

Example:

The wigs and robes are clearly an anachronism in the legal system.

Related/similar words: absolute, outdated, etc.

Etymology: mid-17^th century

"An error in computing time or finding dates," from Latin anachronismus, from Greek anakhronismos, from anakhronizein "refer to wrong time," from ana "against" (see ana-) + khronos "time" (see chrono-). The meaning "something out of harmony with a specified time" is recorded by 1816.