A straightforward way to define fiscal sustainability is as a situation in which the ratio of federal debt to national income is stable or moving down over the longer term. This goal can be attained by bringing spending, excluding interest payments, roughly in line with revenues, or in other words, by approximately balancing the primary budget.
Bureau of Labor (1889). Fourth Annual Report of the Commissioner of Labor, 1888: Working Women in Large Cities. Washington: Government Printing Office, https://archive.org/details/cu31924002402919. Bureau of Labor Statistics (2016). National Compensation Survey: Employee Benefits in the United States, Table 32: Leave Benefits: Access, Private Industry Workers, March 2016. Washington: BLS, September, https://www.bls.gov/ncs/ebs/benefits/2016/ebbl0059.pdf. Cajner, Tomaz, Dennis Mawhirter, Christopher Nekarda, and David Ratner (2014).