A monorepo is a single repository containing more than one project (in contrast to the single-repository-per-project approach).
This definition is reduced from the Atlassian blog entry:
Definitions vary, but we define a monorepo as follows:
- The repository contains more than one logical project (e.g. an iOS client and a web-application)
- These projects are most likely unrelated, loosely connected or can be connected by other means (e.g via dependency management tools)
(for more, click on their link). Monorepos are not limited to git; any version-control system can hold a monorepo.