A very large number defined in terms of circle notation by Steinhaus (1983) as 
.
Megistron
See also
Mega, Moser, Steinhaus-Moser NotationExplore with Wolfram|Alpha
References
Steinhaus, H. Mathematical Snapshots, 3rd ed. New York: Dover, pp. 28-29, 1999.Referenced on Wolfram|Alpha
MegistronCite this as:
Weisstein, Eric W. "Megistron." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Megistron.html