The Free On-line Dictionary of Computing (30 December 2018):
(csh) The Unix command-line interpreter
shell and script language by William Joy, originating
from Berkeley Unix.
Unix systems up to around Unix Version 7 only had one
shell - the Bourne shell, sh. Csh had better interactive
features, notably command input history, allowing earlier
commands to be recalled and edited (though it was still not as
good as the VMS equivalent of the time).
Presumably, csh's C-like syntax was intended to endear it
to programmers but sadly it lacks some sh features which are
useful for writing shell scripts so you need to know two
different syntaxes for every shell construct.
A plethora of different shells followed csh, e.g. tcsh,
ksh, bash, rc, but sh and csh are the only ones which
are provided with most versions of Unix.