A post which uses an archived version of its original source.
For sourcing purposes, it is generally recommended to leave the now-defunct link as the source, rather than the archived link itself, as to avoid hotlinking complications and putting unnecessary traffic strain on the archival websites. Feel free to mention the use of an archived link in the post's comments, though this isn't strictly necessary.
Do not use this tag to indicate that a source has been archived on an archival website, nor as a substitute for Bad ID; and do not use this tag as an excuse to change defunct sources to archived sources. None of these are not what this tag is for.
