System prompt
Standing instructions sent ahead of every conversation, setting the role, rules and format.
Also written: system message, system instructions.
The system prompt is the part of the context you write once and send every time. It is where the role, the constraints, the output format and the things the model must never do belong.
Two practical notes. It is sent on every single call, so a long one costs you on every turn — which is exactly what prompt caching (Paying a reduced rate for a prefix the vendor has already processed, instead of full price for sending it again.) is for. And it is instructions, not enforcement: a system prompt is a strong steer, not a security boundary.