Secure SHell
SSH and the resulting protocols facilitate access to systems running the SSH server application. This allows a user the run applications on the server, copy files to and from the server, logon to the server (although this is also considered running an application) and more.
One definition:
In computing, Secure Shell or SSH is a set of standards and an associated network protocol that allows establishing a secure channel between a local and a remote computer. It uses public-key cryptography to authenticate the remote computer and (optionally) to allow the remote computer to authenticate the user. SSH provides confidentiality and integrity of data exchanged between the two computers using encryption and message authentication codes (MACs). SSH is typically used to log into a remote machine and execute commands, but it also supports tunneling, forwarding arbitrary TCP ports and X11 connections; it can transfer files using the associated SFTP or SCP protocols.