It's called the complement. But you'd call it this only in certain contexts; those include probability, and percentages (e.g. the complement of the percent reduction is the percent paid).The difference between a value and 1 (i.e. 1-x) is very common in maths, and I was just wondering if it had a name? Similar to 1/x is the reciprocal or inverse (I forget the technical difference, please don't correct me).
Thanks