Fzninety

Fzninety is equal to \(90^{90}\) = \(90\uparrow\uparrow2\) ≈ \(7.67113\times10^{175}\).

Full decimal expansion:

76177348045866392339289727720615561750424801402395196724001565744957137343033038019601000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Etymology
Fzninety is made out of the Fz- prefix and ninety, which ends up becoming \(90^{90}\) when concatenated backwards, or 90 to the power of itself.