A domain-specific vocabulary refers to vocabulary that is specific to a certain area of study, profession, or field. It includes terms, phrases, and jargon that are unique to that discipline and may not be easily understood by people outside of that area. For example, medical terminology for healthcare, legal terms for law, or technical language for engineering are all instances of domain-specific vocabulary.
The other options you mentioned, such as "vocalist that is accessible for readers all ages" or "vocabulary that is interesting and vivid," do not accurately describe the concept of domain-specific vocabulary.