Is there an English word meaning "the use of uncommon words"?
Is there an English word meaning "the use of uncommon words" or similar?
It depends on what you mean by "uncommon". Lexiphanicism is the use of excessive terminology, while patavinity is the use of local or provincial words.
(I hope you are not just looking for slang or jargon.)
In the category of similar words, there's sesquipedalian, meaning prone to using long words.
If the uncommon words are being used specifically to obscure meaning, then "obfuscation" might apply.