System prompt
The standing instructions an operator places ahead of every conversation (tone, rules, role) which the user never types and usually never sees.
The system prompt is ordinary text that sits first in the context window, ahead of your message, on every single turn. The model has no separate channel for "rules": it just reads them first. Two consequences: it costs tokens every request, and it is a request rather than a guarantee, so a determined user can often talk a model into repeating it or ignoring it. Never put a secret in one. Products give it other names, with ChatGPT calling it "Custom Instructions" and Copilot "agent instructions", but it is the same idea: standing text read before your message.
Never put a secret in one. People talk models into repeating them, so we assume anything in there is public. Keeping it stable also keeps the cache warm, which is the cheaper half of the reason.