“Stet” is a Latin word meaning “let it stand.” It’s traditionally used as a proofreader’s mark or direction to a typist or typesetter. The instruction means that a previous change should be ignored and the text should be left in its original condition. Thus, for example, if a (possibly overly aggressive) lawyer ends a letter to an adversary with the statement “Conduct yourself accordingly,” then crosses it out in editing, and then writes “stet” next to it on a later proofreading, it means that she intends for her assistant to keep the original phrase in the finished product.
The use of this word appears to have diminished and in some quarters to have become unknown. This is unfortunate because it is an efficient and useful tool.
— BBC&B