System prompt
The invisible instructions the AI receives before yours.
The analogy
Before an actor walks on stage, the director has given them notes the audience never hears: the role, the tone, the lines they must never say. The system prompt is that briefing: a hidden message that defines how the assistant behaves before you type your first word.
In detail
It's a message with special priority that the developer places at the start of the context: it defines the assistant's identity, rules, tone and boundaries. The model gives it more weight than user messages, even though technically everything is text in the same context. That's why two products built on the same model can behave completely differently.
An example
“You are a bank's assistant. Only answer questions about the bank's products, never give investment advice, and route any complaint to 555-0123.” You only see the chat; these rules work in the shadows.