The chapter head is set in report.cls by the macro \@makechapterhead:
\def\@makechapterhead#1{%
\vspace*{50\p@}%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\huge\bfseries \@chapapp\space \thechapter
\par\nobreak
\vskip 20\p@
\fi
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak
\vskip 40\p@
}}
We can create a "duplicate" of it, only with the creation of Chapter X removed. Let's call this duplicate \fake@makechapterhead:
\def\fake@makechapterhead#1{%
\vspace*{50\p@}%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\huge\bfseries \strut%\@chapapp\space \thechapter
\par\nobreak
\vskip 20\p@
\fi
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak
\vskip 40\p@
}}
The commented part has been replaced by \strut (in order to have an appropriate baseline correctly set.
Now we add some house-keeping and user-interface macros and you can use \newchapterhead to create the "fake" chapter head, and \restorechapterhead to restore the original setting:
\documentclass{report}
\makeatletter
\let\old@makechapterhead\@makechapterhead
% Taken from http://mirrors.ctan.org/macros/latex/unpacked/report.cls
\def\fake@makechapterhead#1{%
\vspace*{50\p@}%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\huge\bfseries \strut%\@chapapp\space \thechapter
\par\nobreak
\vskip 20\p@
\fi
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak
\vskip 40\p@
}}
\newcommand{\newchapterhead}{\let\@makechapterhead\fake@makechapterhead}
\newcommand{\restorechapterhead}{\let\@makechapterhead\old@makechapterhead}
\makeatother
\begin{document}
\chapter{Hello}
\newchapterhead
\chapter{World}
\restorechapterhead
\chapter{!}
\end{document}
The above procedure keeps the same vertical location for the chapter title by merely removing the phrase Chapter X from being printed.
An alternative approach might be to use the \chapter* setting of a chapter. This doesn't provide the same vertical position on the chapter-page for the title, but may also be what you're after:
\documentclass{report}
\begin{document}
\chapter{Hello}
\clearpage\refstepcounter{chapter}%
\chapter*{World}
\chapter{!}
\end{document}
The issuing of a \clearpage\refstepcounter{chapter} allows for two things:
Steps the chapter counter to have a correct reference for sub-content in chapter World;
Correct mark in the document if you're using hyperref for placing a internal document hyperlink.
\chapter*{World}to not have the title, and\stepcounter{chapter}to keep the chapter numbering consistent. – ChrisS Aug 03 '14 at 04:06\refstepcounterrather than\stepcounter, as all kinds of cross-referencing mechanisms rely on counters being stepped via\refstepcounter. – Mico Aug 03 '14 at 04:14\refstepcounterwould be necessary only if one wants to set a label for the (inexistent) number. Otherwise\stepcounteris as good. On the other hand it' difficult to understand why chapter two shouldn't receive its number. – egreg Aug 03 '14 at 09:12