Definition
A false negative occurs when a system predicts that a condition is absent even though the reference says it is present. It is a miss: undetected fraud, a disease not flagged, a relevant document not retrieved, or a failing test judged as passing.
The false-negative rate is false negatives / (true positives + false negatives). It answers: among the actual positive cases, what share did the system miss? The denominator requires trustworthy reference labels.
The operational harm depends on recovery. A missed spam message may be noticed by the user; a missed safety defect may become irreversible. Evaluate false negatives by relevant slice and consequence, not only as an aggregate count.
Distinguish it from nearby terms
A false positive raises an alarm for a condition that is absent. A false negative misses one that is present. Recall is one minus the false-negative rate when both use the same positive class and reference set.
Check your understanding
A content filter misses 2 percent of prohibited items overall but 20 percent in one language. The aggregate false-negative rate hides the population carrying most of the risk.