Recall
Also: sensitivity, true positive rate, TPR, hit rate
True positives divided by all actual positives, TP / (TP + FN). High recall means few misses; it usually trades off against precision as the decision threshold moves.
Recall = TP / (TP + FN)
Example
There are 60 spam emails and the filter catches 40 of them: recall = 40 / 60 ≈ 0.67, so 20 spam emails slip through.