Time.
Nature's way of
making sure Everything doesn't happen at once.
From a UN*X fortune cookie.
Also, a UN*X shell command (although also available in your path, generally as /usr/bin/time or /usr/bin/timex) which times the execution of some command, reporting wallclock time (elapsed time) as well time spent by the program in user mode and system mode (kernel mode), sometimes a bit more data.