Quadrillion

According to the French notation, which is followed also upon the Continent and in the United States, a unit with fifteen ciphers annexed; according to the English notation, the number produced by involving a million to the fourth power, or the number represented by a unit with twenty-four ciphers annexed. See the Note under Numeration.

the number that is represented as a one followed by 15 zeros

the number that is represented as a one followed by 24 zeros in the UK; "in England they call a septillion a ‘quadrillion’"