tcsh (pronounced “T-shell”) is a Unix shell based on and compatible with the C shell (csh). It is essentially the C shell with (programmable) filename completion, command-line editing, and a few other features.
The ‘t’ in tcsh comes from the T in TENEX, an operating system which inspired Ken Greer, the author of tcsh, with its command-completion feature. Ken Greer worked on tcsh in the late 1970s at Carnegie Mellon University. Paul Placeway from Ohio State University continued work on it in the 1980s, and since then it has been maintained by numerous people. Wilfredo Sanchez, the former lead engineer of Mac OS X, worked on tcsh in the early 1990s at MIT.
Early versions of Mac OS X shipped with tcsh as the default shell, but it has since been replaced by bash.