Why is the space of sections of $E$ called $\Gamma(E)$?

The space of sections of a bundle $\pi: E \to B$ is commonly denoted $\Gamma(E)$. (Note that the graph of a function $f$ is $\Gamma(f):=\left\{(x,f(x))\right\}$, and a particular section $\sigma: B \to E$ "is" a graph $\left\{(b,\sigma(b)) : b \in B\right\}$; but it is the space of all such objects that is called $\Gamma(E)$. I find this a bit confusing, etymologically.) If knowable, what is the reason for this letter (and where and by whom was this notation introduced)?