IPL (initial program load) is a mainframe term for the loading of the operating system into the computer's main memory . A mainframe operating system (such as OS/390) contains many megabytes of code that is customized by each installation, requiring some time to load the code into the memory. On a personal computer, booting or re-booting (re-starting) is the equivalent to IPLing (the term is also used as a verb) .
In earlier operating systems, when you added devices to the hardware system, you had to stop the system, change the configuration file, and then "re-IPL," an activity that meant the system would be unavailable for some period of time. Today's systems provide dynamic reconfiguration so that the system can keep running.