APA Dictionary of Psychology

an index, typically a correlation coefficient, that reflects how well an assessment instrument predicts a well-accepted indicator of a given concept or criterion. For example, if a measure of criminal behavior is valid, then it should be possible to use it to predict whether a person (a) will be arrested in the future for a criminal violation, (b) is currently breaking the law, and (c) has a previous criminal record. A validity coefficient could be used to relate scores on the measure to each of these criteria and thus determine how useful the measure actually is for behavioral forecasting.