System prompt
The instruction that sits above the conversation and applies to every turn in it: who the model is, what it may do, and what it must refuse.
It is not a secret and should not be treated as one. A determined user can get any of it out, so a rule that matters has to exist where it is enforced as well — in the tool that refuses, in the permission that was never granted — and not only in the sentence that asks.
Length works against it. A system prompt grown to several pages competes with itself: later instructions collide with earlier ones and the model follows whichever reads as more specific. Rules that apply sometimes belong in the turn where they apply, not at the top forever.
It is also the part billed on every single call, which is the whole reason prompt caching exists.