What does "Pure" mean, in the context of programming languages and paradigms?

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In other words, what makes a language pure?

For example, Smalltalk is considered to be a purely object-oriented language. Haskell and Lisp are arguably known to be purely functional languages.

When we say pure, does that mean they're not capable of other programming paradigms (which is far from the truth) or does it mean they were designed to be used in that "purely" X paradigm?

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