Nomothetic means, of or pertaining to the study or discovery of general laws. Nomothetic means people's unique personalities can be understood as them having relatively greater or lesser amounts of traits that are consistently across people. Nomothetic explanation describes the effort to derive laws that explain types or categories of objective phenomena. Nomothetic explanation is based on what Immanuel Kant described as a tendency to generalize, and is typical for the natural sciences.
Examples of nomothetic explanation: Why do men murder their partners? Or, What are the causes of international violence? These explanations are known as nomothetic explanations.
Idiographic means people have unique personality structures, thus some traits are more important in understanding the structure of some people than others. However the whole issue of whether a trait exists in all people to a greater or lesser degree is complicated by different views of the trait perspective.
There are two different views as to whether all traits exist in all people: Nomothetic literally means "proposition of the law" and is used in philosophy, psychology, and law with differing meanings.
In psychology, nomothetic measures are contrasted to ipsative or idiothetic measures, where nomothetic measures are measures that are observed on a relatively large sample and have a more general outlook while the idiographic approach is relating to a more singular case as is done in case studies.
The Nomothetic view emphasizes comparability among individuals but sees people as unique in their combination of traits. This viewpoint sees traits as having the same psychological meaning in everyone.
The belief is that people differ only in the amount of each trait. It is this which constitutes their uniqueness. This approach tends to use self-report personality questions, factor analysis etc. People differ in their positions along a continuum in the same set of traits.
Most contemporary psychologists tend towards a nomothetic approach (and the trait approach is often viewed solely as a nomothetic approach these days), but they are aware of how a trait may be slightly different from person to person in the way that it is expressed.