System prompt
The initial instruction an LLM receives that sets its role, tone and rules — before the user starts talking.
"You are a legal assistant. Answer concisely. Always cite sources. Never speculate on jurisdiction." The user doesn't see this; it's like stage directions behind the curtain.
Good system prompts are specific about what the model should do, what it should not do, and in what tone. The default is often "be helpful, harmless, honest" — which is vague.