An Acronym for Secure Shell.
A tool provides strongly encrypted / authenticated communications. It is mainly used to provide shell access on remote systems but also has a lot more to offer – an attractive replacement for less secure access methods like rsh(1) and telnet(1).
The original and subsequent versions of SSH were released under more-and-more restrictive licenses until it was deemed to be non-Free, so the OpenSSH Fork was started with the last “good” Free version (which remains under the BSDLicense). These days, OpenSSH is the version used in Linux and BSD distributions, although it is merely called SSH. There’s now also a mildly popular alternative implementation under the ægis of the GNU project, called LSH.