Prime in text mode

If you really want the math prime symbol you may simply use $'$ or $\prime$, or if you want to use it in text and math mode \ensuremath{'} or \ensuremath{\prime}, e.g.:

\documentclass{article}
\newcommand*{\everymodeprime}{\ensuremath{\prime}}
\begin{document}
Prime in text mode: \everymodeprime

Prime in math mode: $\everymodeprime$
\end{document}

I've used \prime instead of ' to avoid typo mistakes.

\ensuremath does only switch to math mode, if math mode it not already active. So it is often useful to define commands, that uses math mode commands and should work in text and math mode.

The flexisym package offers \textprime which can be directly used in math or in text mode:

\documentclass{article}
\usepackage{flexisym}

\begin{document}

a\textprime

$a\textprime$

\end{document}

I don't think there's a \prime character in text mode. If you want an upright single quote, you can use \textquotesingle defined in the textcomp package; alternatively if you actually want a prime, then you can just use $'$ in text mode.