Wiktionary
cross-multiplication
n. (context mathematics English) An instance, or the process, of cross-multiplying.
Wikipedia
Cross-multiplication
In mathematics, specifically in elementary arithmetic and elementary algebra, given an equation between two fractions or rational expressions, one can cross-multiply to simplify the equation or determine the value of a variable.
Given an equation like:
$$\frac a b = \frac c d$$
(where and are not zero), one can cross-multiply to get:
$$ad = bc \qquad \mathrm{or} \qquad a = \frac {bc} d.$$
In Euclidean geometry the same calculation can be achieved by considering the ratios as those of similar triangles.