General synonyms:
* Technical language
* Slang
* Cant
* Vocabulary (specific to a group)
* Lingo
* Terminology
* Idiom (more specifically refers to phrases, not words)
* Argot (often used for criminal slang)
More formal synonyms:
* Specialized language
* Professional language
* Trade language
* Technical terminology
Informal synonyms:
* Gibberish (implies incomprehensibility)
* Gobledygook (similar to gibberish, often humorous)
* Shop talk
* In-group talk
The best synonym depends on the specific situation and the nuance you're trying to convey.