Precision measures how trustworthy a system's alerts are: of all the events it flagged, what fraction were real. Formally it is true positives divided by all positives the system raised — true positives over true positives plus false positives. A precision of 0.9 means nine in ten alerts were genuine and one was a false alarm. It is the metric that answers an operator's gut question: "when this thing cries wolf, how often is there actually a wolf?"
Precision is one half of the accuracy picture, and it cannot be read alone — its partner is recall, which measures how many real events were caught. The two trade off: you can make a detector extremely precise by only alerting when it is absolutely sure, but then it stays silent on borderline real events (low recall). Quoting a single "accuracy" number hides this trade; a serious specification gives precision and recall together for the actual operating conditions.
The pitfall is optimising precision in a vacuum. High precision with terrible recall is a detector that almost never false-alarms because it almost never alerts at all — useless for catching events. Which way to lean depends on the cost of the two errors: a watchlist or retail alert wants high precision (don't accuse the innocent), a perimeter at night wants high recall (don't miss the intruder), and you set the confidence threshold to pick the balance. Precision is a range tied to conditions and tuning — never 100% in the real world.
