The Tax Gap is the difference between the amount of tax that should, in theory, be collected by the WRA (the theoretical liability), and what is collected.
Y Bwlch Treth yw'r gwahaniaeth rhwng swm y dreth a ddylai, mewn theori, gael ei chasglu gan yr Awdurdod (yr atebolrwydd damcaniaethol), a’r hyn a gesglir.