Answer
$\frac{1}{x}$
Work Step by Step
Let's assume that reciprocal of $ x$ is '$R$'.
By definition, product of a number and its reciprocal is always 1. Therefore-
$ x \times R$ = 1
or $ R$ = $\frac{1}{x}$
i.e. reciprocal of $x$ is $\frac{1}{x}$