FreeS/WAN - Definition from the Hotspot Shield Glossary
Free Swan is an implementation of IPsec (Internet Protocol Security) and allows secure tunnels to be built through public and untrusted networks. FreeS/WAN is a Linux implementation and uses a combination of strong cryptography and encryption to ensure that packets of data are not tampered with or altered in any way, and come from the correct, trusted source.
S/WAN was realized by John Gilmore in 1999 and has been developed over the years with several different release versions that are free to download. Although FreeS/WAN is no longer in active development the core code and software is still available for download and modifications and is still used today on Linux devices.