What is a False Negative?
Internet Marketing Dictionary | False Negatives

False negatives are outcomes where a test reports something as not being the case whereas in fact it is. For example, a rain detected may indicate that it is not raining when in fact it is.

False negatives compare with false positives where, to follow our example, the rain detector may say that it is raining when in fact it is not.

In statistics false negatives are also referred to as Type 2 errors.

A situation where most internet marketers meet problems with false negatives is in spam filtering. An effective spam filter must stop all spam and let through all valid emails. A spam filter stopping a legitimate email is an example of a false negative. By setting very tight filtering levels, the number of false negatives will increase. Conversely by setting filtering levels too low, false negatives will diminish but false positives will rise - more spam will get through.