A metric unit of frequency representing 10^-18 cycles per second. Because one hertz is one cycle per second, a single cycle at this frequency would take roughly 31.7 billion years to complete. It is a strictly theoretical unit used in cosmology to describe cyclic phenomena that operate on timescales longer than the current age of the universe.
Heard on the job
You'll only see attohertz used in theoretical papers discussing the expansion and contraction cycles of the universe.