Graham's number

Graham's number is a... rather large number. Letting $f\left(x\right)=3{\uparrow }^{n}3$ (in Knuth_up_arrow_notation) and $f^n\left(x\right)=\underset{n\text{applications of}f}{\underset{}{f\left(f\right(f(\cdots f(f\left(x\right))\cdots ))}}$, Graham's number is defined to be $f^{64}\left(4\right).$

The result is sizable. For an explanation of how large this is and why, see Tim Urban's explanation at Wait but Why.