(number theory, field theory) a norm for the rational numbers, with some prime numberp as parameter, such that any rational number of the form — where a, b, and p are coprime and a, b, and k are integers — is mapped to the rational number , and 0 is mapped to 0. (Note: any rational number, except 0, can be reduced to such a form.) 
According to Ostrowski's theorem, only three kinds of norms are possible for the set of real numbers: the trivial absolute value, the real absolute value, and the p-adic absolute value.WP
A notation for the p-adic absolute value of rational number x is .
The function is actually from the set of rational numbers to the set of real numbers, because it is used to construct/define a completion of the set of real numbers, namely, the field of p-adic numbers, and this field inherits this p-adic absolute value and extends it to apply to p-adic irrationals, which could well be mapped to real numbers in general (not merely rationals).