A large language model version released between major generations.
GPT-5.5 is like a video game patch. No new castle, but the lag and weird buttons feel less annoying.
You may meet it in chat helpers and coding tools. It is mostly polish, not a whole new AI idea.
GPT
GPT-5.5 belongs to the GPT family as a between-generation version.
LLM
GPT-5.5 is still an LLM, just with a finer version name.
ChatGPT
Models like GPT-5.5 often power chat products and shape daily use.
Frontier model
If GPT-5.5 leads the field, people may call it a frontier model.