Rerun a previously entered command again in a different context.
First run, count all filesystem objects under the /tmp path:
user@host:~$ find /tmp | wc -l 1376
Rerun the same command, but this time in the context of the time command in order to measure how long it takes to complete:
user@host:~$ time !! time find /tmp/ | wc -l 1376 real 0m0.008s user 0m0.000s sys 0m0.004s