System prompt
Also called system message, system instructions, custom instructions
Standing instructions given to an AI model before the conversation starts, which shape every reply.
A system prompt is a block of instructions placed at the start of a conversation, separate from the user's messages. Models are trained to treat it as higher-priority guidance: who to act as, what rules to follow, what format to use and what to avoid. It stays in effect for the whole conversation.
Every AI product you use has one. ChatGPT, Claude and Gemini apps each send a system prompt describing the assistant's behaviour, the date, and available tools. Companies building on these models write their own, often hundreds or thousands of words long, describing their product, policies and tone.
You can write your own in several places. In APIs it is a dedicated system field. In chat apps, features like custom instructions, projects or custom GPTs act as personal system prompts that apply to every chat. A good system prompt focuses on durable rules and facts. Task-specific details belong in the user message.
Example
A system prompt for a recipe app might say: "You are a cooking assistant for home cooks in the UK. Use metric units and grams. Suggest substitutions for common allergens. If a question is not about food or cooking, say you can only help with cooking."
Common mistake
Treating the system prompt as a security boundary. Users can sometimes talk a model into ignoring or revealing it, which is what jailbreaks and prompt injection exploit. Never put secrets such as API keys in a system prompt, and enforce important rules in code as well.