The nomothetic approach studies personality by looking for general laws and dimensions that hold across people, measuring everyone on the same scale so individuals can be meaningfully compared to one another.
It's the logic behind almost every personality test you've ever taken, including the Big Five. When a report tells you that you score in the 80th percentile on Extraversion, it's making a nomothetic claim: it's placing you on a dimension that's assumed to apply to everyone, and ranking you against a reference group. The approach doesn't try to capture what makes you singularly you — it tries to find the traits and patterns general enough to describe anyone, then locate each person's position on them.
This term comes from psychologist Gordon Allport, who contrasted it with the idiographic approach: studying the unique configuration of a single person's life rather than where they fall on shared dimensions. Neither approach is "correct" on its own — they answer different questions. Nomothetic research is what makes group comparisons, prediction, and standardized testing possible; idiographic research captures the texture of an individual life that group-level averages can wash out.
Most commercial and research personality assessment, TraitLab included, is nomothetic by design: the value of a Big Five score comes precisely from its being calibrated against a large population, which is what makes the comparison meaningful in the first place.