The Jargon File (version 4.4.7, 29 Dec 2003):

backreference
 n.

    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??