System prompt
Also known as: system message, custom instructions
A system prompt is a background instruction that sets an AI's role, tone, and rules for an entire conversation, above the individual messages you send.
In more detail
Many AI tools let you set persistent instructions — sometimes called a system prompt or 'custom instructions' — that apply to every message in a chat: who the assistant should act as, what format to use, what to avoid. It's how you avoid repeating 'answer in British English, be concise' in every single message. In consumer apps this often appears as a settings field; in developer APIs it's an explicit system message.
Example
Setting a system prompt like "You are a careful proofreader. Only fix grammar and clarity; never change my meaning." makes every later message in that chat follow those rules without restating them.
Why it matters
A good system prompt saves repetition and makes AI output consistent — useful when you do the same kind of task over and over.
Prompts that use this
Try a prompt for yourself
Open any prompt and run it in the IterPrompt playground.