The Ubiquitous Language is a concept in Domain Driven Design that describes the language that is used by Domain Experts and Developers alike when speaking / arguing about any aspect of the domain or the application.
Domain Experts being the people who are knowledgeable about the domain / business, that is the components, processes, actors and all their interrelations.
The goal of the Ubiquitous Language is to assure that (business) owners and developers understand each other; that the resulting software actually addresses the problems; that communication is truly bidirectional.
The Ubiquitous Language emerges iteratively from discussion, thinking, feedback.
If the Ubiquitous Language changes, then all (code, documentation, etc) must change.