System prompt
The instruction block that frames a conversation, separate from the user's messages.
By Lattice editorial · Entry last reviewed · method
What it implies
Two things about it are easy to get wrong. It is usually where you put behaviour instructions that belong in a validation function, where they will be followed unreliably. And if it is stable, splitting it out and reusing it verbatim enables prefix caching, which is often the single largest cost win available.