How to highlight keywords?

With XeLaTeX it's possible via the xesearch package.

\documentclass[border=1]{standalone}
\usepackage{fontspec}
\usepackage{xesearch}

\SearchList{list1}{\textit{#1}}{keyword,something}

\begin{document}\Huge
This is text that shows a keyword,\\
which is something special.
\end{document}

The result is shown below

No, LaTeX can't search through the whole document for you to highlight specific words. You need to format them by yourself. You might want to define a custom macro for this which you can change later:

\newcommand*{\kw}[1]{\emph{#1}}
%\newcommand*{\kw}[1]{\emph{#1}\footnote{#1: ...}

You could use the Search&Replace feature of your Text/LaTeX editor to add this macro around keywords. Using tools like sed or Perl you could do this for all words in your CSV file at once.

For showing the definition of the keyword you can use the glossaries or acronym packages.