Exploratory testing
Exploratory testing is a structured but unscripted form of manual testing where a tester simultaneously designs and executes tests, using their understanding of the product to probe for failures. The approach was codified by Cem Kaner in 1984 and championed by James Bach in the 2000s; it's the dominant approach in organisations that have moved past 'follow this test script' to skilled testing.
Exploratory testing's value is finding the bugs that scripted tests don't — usability issues, surprising edge cases, integration gaps. A skilled exploratory tester forms hypotheses, designs charters ('explore the checkout flow with edge-case payment data, 90 minutes'), and produces session notes that feed back into the test suite. It's not 'manual testing' in the disparaging sense — it's a specific discipline with techniques (session-based test management, heuristic test strategies, the Heuristic Test Strategy Model). Modern engineering organisations layer exploratory testing on top of automated regression suites: automation catches the known regressions, exploration catches the unknown new defects.