I was recently reading an answer to this question, and I was struck by the statement "The language is mature".

So I was wondering what we actually mean when we say that "A programming language is mature"? Normally, a programming language is initially developed out of a need, e.g.

• Try out / implement a new programming paradigm or a new combination of features that cannot be found in existing languages.
• Try to solve a problem or overcome a limitation of an existing language.
• Create a language for teaching programming.
• Create a language that solves a particular class of problems (e.g. concurrency).
• Create a language and an API for a special application field, e.g. the web (in this case the language might reuse a well-known paradigm, but the whole API must be new).
• Create a language to push your competitor out of the market (in this case the creator might want the new language to be very similar to an existing one, in order to attract developers to the new programming language and platform).

Regardless of what the original motivation and scenario in which a language has been created, eventually some languages are considered mature. In my intuition, this means that the language has achieved (at least one of) its goals, e.g. "We can now use language X as a reliable tool for writing web applications."

This is however a bit vague, so I wanted to ask what you consider the most important criteria (if any) that are applied when saying that a language is mature.

IMPORTANT NOTE

This question is (on purpose) language-agnostic because I am only interested in general criteria. Please write only language-agnostic answers and comments! I am not asking whether any specific "language X is mature" or "which programming languages can be considered mature", or whether "language X is more mature than language Y": please avoid posting any opinions or reference about any specific languages because these are out of the scope of this question.

EDIT

To make the question more precise, by criteria I mean such things as "tool support", "adoption by the industry", "stability", "rich API", "large user community", "successful application record", "standardization", "clean and uniform semantics", and so on.