Standing instructions for the AI. The 'rules of engagement' baked in before the conversation starts.
A system prompt is a message at the very beginning of a chat that sets the AI's role, tone, constraints, and goals. The user never sees it. It persists for the whole conversation. It's the difference between a generic assistant and a focused specialist. Think of it as the job description the AI is working from.
The system prompt is where most production behavior gets defined. Changing one line can completely transform your agent. Most people underinvest in their system prompt and then wonder why their AI is inconsistent.