The escalation tool (PagerDuty, OpsGenie, Incident.io) that wakes the on-call engineer when something needs immediate attention.
A pager (in modern usage) is the escalation system that turns an alert into a phone call, push notification, or SMS for the on-call engineer. PagerDuty, OpsGenie, Incident.io, FireHydrant, and equivalent tools handle rotation schedules, escalation policies (page primary, then secondary, then manager), acknowledgment tracking, and integration with chat and incident-management workflows. The 'pager' is now mostly a phone, but the legacy term stuck.
The pager is where reliability theory meets human reality: if the page doesn't reach a person who can act in the time MTTR demands, every other reliability investment is moot. Tuning the pager (escalation timing, ack windows, contact methods, who's notified) is unglamorous work that has dramatic impact on real-world outage duration.
See the part of the platform that handles pager in production.