Psychometrics
The field of measuring psychological traits, and the standards (reliability and validity) used to judge whether a test actually works.
Psychometrics is the science of designing and evaluating tests of ability, knowledge, and personality. Two standards matter most: reliability (do you get a similar score if you retake the test?) and validity (does the score measure what it claims to?).
Instruments used in published research, like the IPIP-NEO behind our Big Five test, are built and studied against both standards. A lot of casual online quizzes are not, which is why it's worth checking what's actually behind a test before treating its result as meaningful.