Lossy data is information that has been reduced or compressed so some original detail is no longer available. In security, logs, alerts, and summaries are often lossy because they preserve the main event but omit timing, sequence, payloads, or surrounding system state.
This matters because cyber defense depends on context. A single alert may look harmless or suspicious depending on the process chain, network path, identity data, or host behavior around it. Attackers can exploit these gaps by blending into normal activity, while defenders can miss multi-step intrusion paths if they rely on compressed records alone. Strong detection and incident response usually require richer telemetry and correlation so a lossy record can be interpreted correctly.