What does $m(I)$ stand for in the following proof?

Question

In the following proof, it is shown that $f_n$ converging to $f$ uniformly implies convergence in $L^{2}$.

If $f_n$ converges to $f$ uniformly (i.e. $\sup_{x \in I}|f_n(x)-f(x)| \to 0$), then $$\|f_n-f\|^2_2=\int_I |f_n(x)-f(x)|^2\,dx \leq m(I) \left(\sup_{x \in I}|f_n(x)-f(x)|\right)^2 \to 0,$$ so $f_n$ converges to $f$ in $L^2$.

I was wondering what $m(I)$ stands for?

Answer 1

$m(I)$ may indicate the measure of interval $I$. For example, if $I=[a,b]$, then $m(I)=b-a$.