Nygard's working definitions anchor the whole book. A **transaction** is an abstract unit of work the system processes — not a database transaction; one transaction ("customer places order") may span many pages and external integrations. Transactions are *the reason the system exists*. A **system** is the complete, interdependent set of hardware, applications, and services required to process those transactions — from a single app to a sprawling multitier network. A **robust system** keeps processing transactions even when transient *impulses*, persistent *stresses*, or component failures disrupt normal processing. That is what stability really means. The crucial reframing: stability is **not** "your individual servers or applications stay up and running." It is that **the user can still get work done.** A cluster can be green on every node while users are completely blocked (as in the grounded airline). Measure stability by sustained transaction throughput from the user's side, never by component uptime. --- *Source: [[Release It Second Edition]] (Michael T. Nygard, Pragmatic Bookshelf 2018) — Ch 3 — Stabilize Your System*