Just for variety, here's a LuaLaTeX-based solution. Note that it doesn't require you to modify the body of the document in any way. The word to be rendered in bold-italics throughout the document should be listed as the second argument of the string.gsub
function.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{luacode}
\begin{luacode}
function highlight_word ( line )
return string.gsub ( line, "myparticularword", "{\\bfseries\\itshape %0}" )
end
luatexbase.add_to_callback ( "process_input_buffer", highlight_word, "highlight_word" )
\end{luacode}
\begin{document}
An interesting story about myparticularword and its history \dots
\end{document}
\newcommand{\mystrangeword}{\textbf{particularword}\xspace}
for example, use\mystrangeword
then. -- and yes,\xspace
has its issues ;-) – user31729 Oct 7 '15 at 15:41