tl;dr: The Twelve-Factor App is a methodology for building SaaS applications that are portable, scalable, and suitable for cloud deployment, originally synthesized from Heroku's experience with thousands of apps. The twelve factors cover practices like using a single codebase with many deploys, explicit dependency declaration, storing config in environment variables, treating backing services as attached resources, separating build/release/run stages, running stateless processes, and treating logs as event streams. The 2025 revision maintains the language-agnostic principles aimed at minimizing dev/prod divergence and enabling continuous deployment.