A number equal to ten
to the power of a googol. That is, 10^10^100.
If you wanted to see an example of a googolplex, you would have to look at the entirety of the universe on a subatomic level: the estimated number of atoms in the universe is around 10^10^78.
If you tried to print all of the zeros in a googolplex in 1 point font, the result would be a line that could stretch from one side of the visible universe to the other (60-80 billion light years
according to current observation and theory) and possibly make the trip around at least once more, assuming
that there was enough matter in the universe for that process to take place.
And the googolplex is not the last number that we have assigned "googol" or "plex" to.