Any process that creates other processes (that create other processes (that create other processes (...)).  The canonical form is a Unix shell script called "rabbit" containing this line:

rabbit&rabbit

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Hey, I wonder if my mac can survive a rabbit job? I just rebooted it because I tried to print ... might as well try something challenging.

''My bet is that it can. Back in the early 1980s this brought every Unix system I tried it on to its knees, but modern versions seem to have learned a bit about resource allocation. Let us know the results. Windows XP (home edition) kills the owning process after 1216 executions.''

I got a few hundred "./rabbit: fork: Resource temporarily unavailable" messages and had to kill the terminal window, but "top" running in another window hardly noticed.