System Prompt
A hidden set of instructions given to an AI before your conversation begins, telling it how to behave, what tone to use, and what it should or should not do. Think of it as a briefing note handed to a new assistant on their first day. This behind-the-scenes briefing is called the system prompt.
When you open a chat tool and start typing, you are not the first voice the AI hears. Behind the scenes it has usually been given a set of standing instructions that shape how it responds to everyone. This is the system prompt, and you normally never see it.
Picture a new assistant arriving for their first shift. Before any customer walks in, their manager gives them a quick briefing: be polite, keep answers clear, avoid certain topics, and always sign off warmly. The system prompt does the same job for an AI. It sets the ground rules that stay in place across the whole conversation.
This is different from the prompts you type. Yours are the specific requests in the moment. The system prompt is the steady background guidance that colours every reply. It is often why the same underlying model can feel friendly and chatty in one product and brisk and formal in another.
Most of the time the system prompt is set by whoever built the tool, so you do not need to worry about it. If you ever use AI through a more advanced interface, you may get to write your own, which is one of the ideas explored in prompt engineering.