I am supposing you are talking about the totals when checking out where it says "Discount" in the totals? If so, that is in app/code/core/Mage/Sales/Block/Order/Totals.php.
However, the proper way to change this would be to edit the language file You can either use the translate.csv, or if that isn't working try editing this:
app/locale/en_US/Mage_Checkout.csv
and add the following two lines to the bottom:
"Discount (%s)","Your Text (%s)"
"Discount","Your Text"
That will override the text for your.