General:
* Slang
* Jargon
* Cant
* Lingo
* Vernacular
More specific:
* Secret language: This emphasizes the hidden nature of argot, often used by groups to keep their communication private.
* Code: Similar to "secret language," this emphasizes the coded nature of argot.
* Technical jargon: This emphasizes the specialized language used by a specific profession or group.
* Street slang: This specifically refers to the slang used in informal settings, particularly among younger people or in urban areas.
* Subculture language: This emphasizes the language specific to a particular subculture or social group.
The best synonym will depend on the context in which you are using the word "argot".