A system prompt is the fixed set of instructions an AI assistant receives before the user types anything. It sets the assistant's role, tone and limits in every conversation.
The providers of ChatGPT, Copilot and Claude have their own system prompts, which users do not see. Organisations can add their own instructions on top, for example in a custom assistant or a project: who the assistant writes for, which tone it should keep and which sources it should use.
A system prompt applies to every task in the assistant. An ordinary prompt applies to the specific task.
In practice
A communications department can build an assistant with your tone of voice and standing guidelines in the system prompt. Each employee then only needs to describe the task itself. It requires someone to own the instructions and keep them up to date when the strategy or guidelines change.
The concepts in this glossary run through my AI courses and workshops, where we work with them on your own tasks.