SIGALRM
The alarm clock: alarm() winds it up and the kernel rings on schedule.
The process winds it with alarm(seconds), gets on with life, and when the deadline arrives the kernel rings this bell. It is Unix's original timeout: before select had deadlines and timerfd existed, this is how you gave up waiting.
The detail that bites: each process has only one of these alarm clocks — a new alarm() cancels the previous one — and the sleep(3) of old was implemented with it. The timeout(1) utility is the over-the-counter heir of the idea.
$ perl -e 'alarm 2; sleep 30'
Alarm clock
$ echo $?
142
(142 = 128 + 14: the alarm clock beat the nap)