A user-defined HTTP callback fired by one system to notify another that an event happened, the universal integration glue.
A webhook is an HTTP POST request that one system sends to a URL the other system has registered, on the occurrence of some event (payment succeeded, deploy finished, alert fired, PR merged). Webhooks are the dominant integration pattern between modern SaaS tools because they're simple (a URL and a body), unidirectional (no polling required), and language-agnostic. Production-grade webhooks include retries with exponential backoff and jitter, idempotency keys, signed payloads (HMAC) so the receiver can verify origin, and a dead-letter path for unrecoverable failures.
Most modern alerting, CI/CD, and incident response chains are wired together with webhooks: Datadog fires an alert, the webhook hits PagerDuty, PagerDuty fires another that hits Slack, an automation hits Jira. Reliability of the whole chain is bounded by reliability of the webhook delivery, which is why idempotency, retries, and signing matter, an unreliable webhook delivery turns a missed alert into a missed incident.
See the part of the platform that handles webhook in production.