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Trackbacks are used by blogs to inform other blogs about entires on or referring to the other blog's entries, like an automatic comment. The specification was first conceived by Six Apart who implemented it in Movable Type.

[edit] TrackBacks and LiveJournal

LiveJournal doesn't have TrackBacks. crschmidt did make an implementation (crschmidt), but it wasn't used. When the Six Apart Acquisition happened, brad made a post to here (news) that said:

Does this mean LiveJournal will finally get TrackBack support? I just knew somebody would ask this! Yes, LiveJournal will get TrackBack. Actually we'd been planning it anyway, and we've even done a partial implementation recently. This kinda bumps up its priority though. :-) In all fairness, there have been a number of TrackBack patches written for the LiveJournal codebase in the past, but none that were usable or scalable enough to run on the main site. Our new version was designed from the beginning to scale.

Of course, Mena at the same time said also that they weren't going to "plaster their sites with advertisements", but LiveJournal got ads before it got TrackBacks.

This is probably because:

• TrackBacks are not a wanted feature for most LiveJournal users, who don't know what they are and probably don't care whether or not they can use them. If they're implemented, you can bet the first cries would be for an opt out.
• TrackBacks have associated spam issues. This has been confirmed by brad's comment here (lj_dev).

TrackBacks were last suggested in April (suggestions), and marked as deferred. There are currently no known plans to implement them at any time in the near future.