"In the past, Congress has regularly acted to extend expired or expiring temporary tax provisions. Collectively, these temporary tax provisions are often referred to as 'tax extenders.' Most recently, in December 2015, Congress addressed tax extenders in the Protecting Americans from Tax Hikes Act of 2015 (PATH Act), enacted as Division Q of the Consolidated Appropriations Act, 2016 (P.L. 114-113). This legislation extended all of the 52 provisions that had expired at the end of 2014. Unlike past tax extenders legislation, however, a number of provisions that had expired at the end of 2014 were made permanent. Several others were extended through 2019. Many provisions were temporarily extended for two years, through 2016."
CRS Report for Congress, R44677
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html