Quadrillion may mean either of the two numbers (see long and short scales for more detail):

1,000,000,000,000,000 (one thousand million million; <math>10^{15}</math>; SI prefix peta) - increasingly common meaning in English language usage.

1,000,000,000,000,000,000,000,000 (<math>10^{24}</math>; SI prefix yotta) - increasingly rare meaning in English language usage.