From LQWiki

Backreference can mean:

1. In a regular expression or pattern match, the text which was matched within grouping parentheses.
2. The part of the pattern which refers back to the matched text.
3. By extension, anything which refers back to something which has been seen or discussed before. “When you said ‘she’ just now, who were you backreferencing?”

This article is based, in whole or in part, on entry or entries in the Jargon File.