Multicolumn/multirow inside align

To get "multicolumn" in align (as asked in the title), I've found that using \omit\rlap{} gives the desired result.

\documentclass{minimal}
\usepackage{amsmath}
\begin{document}

\begin{align*}
\omit\rlap{Formula} && \text{Description} \\
y &= x + 14 & \text{A linear formula} \\
\end{align*}

\end{document}

Output:

Instead of using \rlap, you can use \multispan2{$\displaystyle contents$\hfil}

\omit and \multispan are the plain Tex equivalents of \multicolumn. \omit just removes all of the formatting that {align} puts in and lets you format the table cell as you wish. Multispan gives you a true two-column cell (so if your two-column cell is the widest thing in the table it will set the column width properly) but you have to tell TeX how to format the cell contents afterwards.

The \hfil moves the cell contents over to the left. The $s make it in math mode. The \displaystyle puts it in display math mode--if you have a \sum or \int in your contents, it will be in inline text style (too small) unless you remind TeX that you are in a displayed math environment.

You used the argument syntax of \multirow but wrote \multicolumn (and didn't include the multirow package):

\documentclass{minimal}
\usepackage{multirow}
\usepackage{amsmath,amssymb}

\begin{document}

\begin{align}
\begin{array}{ccccc}
1 & 1 & 3 & \cdots & \multirow{2}{*}{4}\\
1 & 1 & 3 & \cdots &\\
1 & 1 & 3 & \cdots &\\
1 & 1 & 3 & \cdots &\\
1 & 1 & 3 & \cdots &\\
\end{array}
\end{align}

\end{document}