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

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?

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}
$c\,dt = \Longunderstack[l]{average probability that a particular 1-2\cr molecular pair will react according to R\cr in the next infinitesimal time interval dt.}$