Disk space used by the kernel as "virtual" RAM. It is slower than RAM, but because disk space is cheaper, swap is usually more plentiful. Swap space is useful to the kernel for holding lesser-used data and as a fallback when physical RAM is exhausted.
A collection of swap areas used to store the contents of stack and data memory pages temporarily while they are used by other processes.
Disk space used as virtual memory. Swap space can be on a local disk or a remote disk that is accessed via the network.
Space on a disk in which the system can write portions of memory to. Usually this is a dedicated partition, but it may also be a swapfile.