A means by which a website requests notification when a document within the site is linked from an external site. Trackbacks are typically used between blogs and require that both blogs be trackback-enabled to function.
Basically, when Blog B links to Blog A, Blog B sends a trackback to Blog A (usually consisting of the link and an excerpt of the text surrounding the link). Blog A may then display the trackback along with the post comments.
Since there is no automatic verification process for trackbacks, they are subject to abuse by spammers and should be checked before posting.
Note: Some platforms create a special trackback URL for posts which must be used to initiate the trackback.