In a virtual memory system, the condition where a page of memory (or sometimes, perhaps in a UNIX system, an entire process) is not currently in physical memory. Instead, it has been moved out to swap space.
It can also refer to a person. For example, if your coworker has temporarily left the office and your manager comes by looking for him/her, your response could be: "He's paged out at the moment."
See also: page out.