Is there any way I can define a variable in LaTeX?

add the following to you preamble:

\newcommand{\newCommandName}{text to insert}

Then you can just use \newCommandName{} in the text

For more info on \newcommand, see e.g. wikibooks

Example:

\documentclass{article}
\newcommand\x{30}
\begin{document}
\x
\end{document}

Output:

30