the number that is represented as a one followed by 24 zeros
"in England they call a septillion a quadrillion"
the number that is represented as a one followed by 15 zeros
Any very large number, exceeding normal description.
A thousand trillion (1,000 × 1,000^4): 1 followed by fifteen zeros, 10¹⁵.
The word comes from the French quadrillion, which combines the prefix meaning "four" with a suffix used for large numbers. It originally referred to a specific magnitude based on that numerical root.