Originally: the cardinal number equal to the tenth power of a million, represented by 1 followed by sixty zeros (1060) (now chiefly British). Later also (originally US): the cardinal number equal to the eleventh power of a thousand, or 1 followed by thirty-three zeros (1033). Also hyperbolically: an unspecified but very large number or amount.
Early 18th century. From deci- + -llion.