The expected time to break a cipher with a given resource. For example, 12 MIPS years would be the amount of work that one computer, with the capability of processing a million instructions per second, could do in 12 years. The same amount of work could be done by 12 such computers in one year, assuming that the algorithm being executed can be sufficiently parallelized.
NIST SP 800-57 Part 1 Rev. 3 [Superseded]