A very, very large integer
, named after mathematician Leo Moser
The moser is most easily described via some recursively-defined notation: first, adopt the convention that an integer written inside a triangle denotes the inner number raised to its own power. For example, writing "3" inside a triangle is shorthand for 3^3 or 27. Next, we introduce a recursive rule which states that an integer i written inside an (n+1)-gon equals the same integer written inside i concentric n-gons. For example, a "3" inside a square means the same as a "3" inside 3 nested triangles, which we know is the same as "27" inside 2 nested triangles, or 27 ^27 inside a single triangle; thus a "3" in a square equals (27^27)^(27^27), an already largish number.
Call the number represented by "2" inside a pentagon (which equals a "2" inside 2 nested squares) a "mega". Then if we draw a "megagon" (a polygon with a mega sides) around a "2", the number so described is a "moser".
For those (undoubtedly the majority of) readers who were wondering, its last digit is a 6.