In tasks involving binary classification, sensitivity and specificity are respectively the probabilities that a positive or negative outcome will be detected.
We define sensitivity, also known as recall, as
Likewise, we define specificity as
Sensitivity and specificity should not be confused with PPV and NPV:
Sensitivity tells us the probability, given an actual positive outcome, that we will predict it as such. Positive predictive value tells us the probability, given a positive prediction, that the prediction is correct.
Specificity tells us the probability, given an actual negative outcome, that we will predict it as such. Negative predictive value tells us the probability, given a negative prediction, that the prediction is correct.