The canonical place, on any Unix-like system, to put a copy of the Korn shell, written by David Korn of AT&T Bell Labs.

It provides the advantages of /bin/csh for interactive use, while staying clear of its broken design and remaining compatible with /bin/sh. Consequently, it was the favourite shell for many power users on Unix systems throughout the 80s. Unfortunately, it was proprietary code, and it only came with commercial SysV-based Unix (which is codified in [POSIX} standards).

When the 1993 Korn shell version was put in the public domain, an independent attempt to recreate ksh, bash, has already gained serious popularity. Meanwhile, ksh development continues, as do the POSIX standards. As a consequence, if you want to find a Korn-compatible shell there are three names to look for: ksh, pdksh, and bash.