Takkanot are rabbinic decrees that sometimes rather dramatically break with previous tradition or rulings due to new circumstances or new understandings of Jewish law. here is the wikipedia article:
Wikipedia: TakkanahI first learned about the term at a class taught by my sponsoring rabbi about the Conservative ruling on driving to shul on Shabbat which was a "takkanah". In fact, many of the Conservative responsa which seem to go against traditional Jewish law are "takkanot". See the section "Takkanot: Significant legislative changes in Jewish law" about halfway down on this webpage:
Committee on Jewish Law and StandardsTakkanot can be used to impose additional stringencies too. Some takkanot that might be of interest to those on this forum are those in the 20th century by Syrian Jewish communities in Buenos Aires and New York which banned conversions and ruled against accepting any converts into the community.
See
Syrian Jews: Attitudes to ConversionAnd some takkanot are observed by all Jews today (not just specific groups such as those noted above). For example, around the year 1000, Rabbeinu Gershom instituted the takkanah which outlawed polygamy even though it is permitted in the Torah.