What is User Acceptance Testing (UAT)?
UAT is real users (or their representatives) verifying the system supports their actual work before go-live — business scenarios, real-world data, the messy cases the requirements never captured. It answers a different question than QA: not "does it meet spec?" but "can we run the business on it?"
In agile, acceptance shifts left — criteria verified every sprint — but a final UAT pass survives in regulated and enterprise contexts, where formal business sign-off is the gate to production.
Worked example
Before a billing system go-live, twelve clerks spend a week processing last month's real invoices in the new system in parallel. QA had passed everything; UAT still finds two killers: the system can't handle the utility's split-meter accounts (3% of customers, 20% of calls), and month-end batch runs collide with the bank's cutoff. Both were spec-compliant. Neither was business-ready — which is exactly the gap UAT exists to catch.