TypeScript module.

TypeScript supports the module keyword for organizing code, but this wording can lead to a confusion with the ECMAScript's module. Since TypeScript v1.5, it has provided us with the alternative keyword namespace, encouraging us to always use namespace instead whenever we write TypeScript these days. See TypeScript v1.5 release note for more details.

Invalid:

module modA {}

declare module modB {}

Valid:

namespace modA {}

// "ambient modules" are allowed
// https://www.typescriptlang.org/docs/handbook/modules.html#ambient-modules
declare module "modB";
declare module "modC" {}