What does "jargon" most nearly mean?

The term "jargon" refers to specialized language or terminology used by a particular group, profession, or field that may not be easily understood by those outside that group. It often includes specific phrases, abbreviations, or technical terms that are familiar to insiders but can be confusing to outsiders.

In this context, "lingo" captures this idea perfectly, as it also denotes a specific type of language or vocabulary associated with a particular group or context, often informal. This makes it synonymous with "jargon," highlighting the shared characteristic of specialized or non-standard language.