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c.1300, "to strike, strike the hands together," possibly imitative of the sound of hitting. The sense of "exchange, barter, trade" is first recorded 1590s, possibly from the practice of slapping hands together as a sign of agreement in bargaining. Related: Swapped; swapping. The noun in this sense is attested from 1620s.
To move a program from fast-access memory to a slow-access memory ("swap out"), or vice versa ("swap in"). The term often refers specifically to the use of a hard disk (or a swap file) as virtual memory or "swap space".
When a program is to be executed, possibly as determined by a scheduler, it is swapped into core for processing; when it can no longer continue executing for some reason, or the scheduler decides its time slice has expired, it is swapped out again.
This contrasts with "paging" systems in which only parts of a program's memory is transfered.