The principle that a legislature cannot do indirectly what it is forbidden to do directly; if it lacks competence over a subject, it cannot achieve the same result by disguising the law under a head it does have power over. Captured by the maxim "what cannot be done directly cannot be done indirectly".
It is a standard federalism and judicial-review doctrine, frequently asked alongside pith and substance and the distribution of legislative powers.
The doctrine is about competence (whether the legislature had power), not about good or bad faith; a law passed with a wrong motive but within competence is valid.
A legislature cannot disguise an ultra vires law as something within its competence; the court examines substance, not form (Gajapati Narayan Deo, 1953).