write a small text in an equation environment after the symbol "=" [duplicate]

Question

I wanto to write something like this

c dt = average probability that a particular 1-2
       molecular pair will react according to R
       in the next infinitesimal time interval dt.

I tried with \\ but it does not work. How can I do such a thing?

Answer 1

Here's a different way not shown in Barbara's suggested alternative. Here, the text is explicitly broken with a end-of-line delimiters, whereas the \parbox alternative is flushed to a box of specified size.

\documentclass[12pt]{article}
\usepackage{stackengine}
\setstackEOL{\cr}
\begin{document}
\[
c\,dt = \Longunderstack[l]{average probability that a particular 1-2\cr
       molecular pair will react according to $R$\cr
       in the next infinitesimal time interval $dt$.}
\]
\end{document}