Nx is a build system and monorepo framework designed for large JavaScript and TypeScript projects. It manages multiple applications and libraries in a single repository, handles task orchestration (running builds, tests, linting in the right order), caches results to avoid redundant work, and visualizes dependencies across packages. Nx is opinionated but powerful: it comes with code generators, plugin ecosystem, and deep integrations with popular frameworks (React, Angular, Next.js, Nest.js).