Corpus, in Latin, translate as “body.”

In the world of AI, it refers to the metaphorical “body,” or collection, of data that was used to train the AI.
This corpus is the material the AI reviews to become intelligent in whatever it was designed for.

Every AI’s corpus will be different, because it is humans who decide what kind of data they want to train an AI on. And the corpus the humans decide to train the AI on will depend on what they want the AI to be proficient in.

To know more about it, here is an article:

Bill Gates, Reddit’s CEO, and other tech leaders are increasingly talking about their “corpus.” Now it’s time to learn what that means.

https://www.fastcompany.com/90916291/what-is-a-corpus-ai-corpora-chatgpt