The book's organizing concept — a continuous cycle of building an MVP, measuring customer response with actionable metrics, and learning whether to persevere or pivot. The loop is both a strategic framework and a philosophy of entrepreneurship: the startup that learns fastest wins.

The MVP is the lean startup's primary tool — not a minimal product but the smallest experiment that can test a fundamental business hypothesis. Ries's insistence that the MVP is about learning, not launching, is the concept's most important and most frequently misunderstood feature.

A structured course correction that preserves accumulated learning while testing a new fundamental hypothesis. The pivot fills a critical gap between 'stay the course' and 'give up' — providing a vocabulary and framework for strategic change that is neither stubbornness nor surrender.

Ries positions himself as a reformed failure — an entrepreneur who built the wrong things for the wrong reasons and learned a better way. His confessional authority gives the prescriptive sections credibility: he is not a consultant theorizing from the sidelines but a practitioner sharing hard-won lessons.

The lean startup's unit of progress — not code shipped, not features built, not revenue earned, but demonstrable evidence that the team has discovered something true about its customers. This concept reframes startup success from output metrics to learning metrics.

A measurement system for startups that replaces traditional financial metrics (which are meaningless in early-stage companies) with learning metrics. Innovation accounting provides the discipline of measurement without the false precision of premature financial projections.