System Prompt
Also called: System Message
A system prompt is the standing set of instructions a deployer gives a model, sent ahead of every conversation and invisible to the people using it. It is where the tone, the subjects the assistant will not discuss, the format of its answers and the things it must refuse are all set. A user prompt is one request; the system prompt applies to every request, which makes it the most consequential text in most deployments and, in most organisations, text nobody has reviewed.
In practice
Ask to read the system prompt of any AI product you buy or build, because it is the operative policy and usually the only place the vendor's refusals and disclaimers are written down. Treat edits to it as changes to a control, with a version history and an approver: one altered line changes what the system says to every customer.