In ecclesiastical legislation, is an unwritten law introduced by the long-continued practise of the people with some consent of the competent ecclesiastical legislator.
This consent is usually tacit.
Custom may be in conformity with the law, when it is said to be the best interpreter of the law (canon 29) or contrary to the law, which it abrogates; or besides and beyond the law, in which case it introduces a new law.
To have the force of law a custom must:
(1) be reasonable - a custom reprobated in the law would be but a corruption of it and unreasonable, but no custom can derogate from the Divine law, whether positive or natural;
(2) it must continue uninterruptedly for at least forty years, unless the law forbids a custom, in which case it prescribes against it only after one hundred years.
Like laws, so too customs may be abrogated by later contrary laws or customs; but unless the law expressly states so, it does not abrogate old and immemorable customs; neither do general laws abolish particular customs.
New Catholic Dictionary