A ubiquitous language is a vocabulary shared by everyone involved in a project, from domain experts to stakeholders, to project managers, to developers.
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If you have to define the Ubiquitous Language (UL), then it is a common language used by developers and users of a specific domain through which the domain can be explained easily.
The ubiquitous language has to be crystal clear so that it brings all the team members on the same page and also translates in such a way that a machine can understand.
A ubiquitous language is a vocabulary shared by everyone involved in a project, from domain experts to stakeholders, to project managers, to developers.