definition of messageID?

I'm trying to track down what this does:

        $messageID = $headers["message-id"];
        if ($messageID) $messageID = substr(substr($messageID, 1), 0,

strlen($messageID)-2); else $messageID = "mid:".$id; $result[$id]->messageID = $messageID; }

I see $messageID in a lot of places, while I see messageID(without a variable $ identifier) in other places.

What is "messageID" for?

Found in: /public_html/webmail/program/lib/imap.inc /public_html/webmail/program/steps/addressbook/func.inc $message_id = $headers->messageID; /public_html/webmail/program/steps/mail/compose.inc $_SESSION['compose']['reply_msgid'] = $REPLY_MESSAGE['headers']->messageID; /public_html/webmail/program/steps/mail/func.inc $message_id = $headers->messageID;