Googolplexian - The largest number:

How largest the definite number?

Googol: The larger number,  '1' followed by one hundred zeros $(i.e \;10^{100})$, which is shown as follows:

$googol\;=\;10^{100}\;=\;$10,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,
                                   000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,
                                   000,000,000,000

A googol is larger than the number of elementary particles in the universe, which amount to only $10$ to the $80^{th}$ power.

Googolplex: The second largest number. A '1' followed by a googol of zeros. Which can be written as:

$googolplex\; = \; 10^{googol}\; = \; 10^{(10^{100})}$

This first volume contains the initial digit one with which googolplex start: 

Googolplexian: The world's largest number. A '1' followed by a googolplex of zeros.

$googolplexian\; = \; 10^{googolplex}\; = \; 10^{10^{(10^{100})}}$

Click here for Googolplexian.