Discuss 'Module Augmentation' and its importance for third-party libraries.

Module augmentation allows you to add type declarations to an existing module's namespace from a separate file.

Many libraries (like Express or Knex) allow for custom plugins that attach data to global objects. Module augmentation allows you to tell the compiler about these additions without editing the 'node_modules' folder. You 'declare' the module name and re-open its interfaces to add new properties. This is vital for maintaining type safety in middleware-heavy applications.