If you are reading about diagnostic tests you will often come across the terms sensitivity and specificity which are a measure of the performance of the test given a particular interpretation (cut-off). If you have trouble remember which is which try thinking about the everyday definitions of the words.
Sensitivity: “ability to perceive; readily and acutely affected by some particular influence”. In other words: how well a test detects the disease. A test that is very sensitive will pick up most of the real cases.
Specificity: “having a special determining quality; peculiar to, characteristic of, something”. In other words how particular the test result is. A test that is very specific will tend to only give positive results for real cases.
Some more on sensitivity and specificity:
D G Altman, J M Bland (1994) Statistics Notes: Diagnostic tests 1: sensitivity and specificity, BMJ 308:1552