1. <operating system> (Or "block") When a process on a multitasking system asks the scheduler to deactivate it until some given external event (e.g. an interrupt or a specified time delay) occurs.
The alternative is to poll or "busy wait" for the event but this uses processing power.
Also used in the phrase "sleep on" (or "block on") some external event, meaning to wait for it.
E.g. the Unix command of the same name which pauses the current process for a given number of seconds.
2. <hardware> To go into partial deactivation to save power.