I know that Fred's definition is sometimes the one that people have in mind when they use the term. But I think there is another definition that may be more useful.

I suggest that we call a moral or ethical system absolute if it is believed to be the one and only true morality. Moral principles and judgments are absolute if they are part of such a system. A person would be called an absolutist if he or she believed in such a system. Any principle or judgment that conflicts with such a system is necessarily false.