Concurrent validity is a form of criterion validity that tests whether a measure correlates with an outcome assessed at roughly the same point in time.
Say a researcher develops a short, five-minute anxiety questionnaire meant to substitute for a much longer clinical interview. To check concurrent validity, they'd give both to the same group of people around the same day and see how closely the short questionnaire's scores line up with the interview's diagnoses. If they track closely, the shorter tool has demonstrated concurrent validity — it can plausibly stand in for the longer, more expensive method without losing much accuracy.
The defining feature is timing: both the test and the criterion are measured at essentially the same moment, which is what separates concurrent validity from predictive validity, where the criterion is measured later. This makes concurrent validity faster and cheaper to study, since researchers don't need to wait months or years to see whether a relationship holds up.
The tradeoff is that concurrent validity doesn't tell you anything about whether a test forecasts the future. A tool can line up well with a same-day criterion and still fail to predict how someone will do down the line, which is why concurrent and predictive validity are usually reported as separate pieces of evidence rather than treated as interchangeable.