Limit order
An instruction to buy or sell only at a stated price or better. It puts certainty of price ahead of certainty of execution, so it can go unfilled.
A limit order names the worst price you will accept and refuses anything past it. A buy limit fills at your limit or lower, a sell limit at your limit or higher, and if the market never reaches that price, nothing happens at all. That is the exchange being made: you give up the certainty a market order provides and take control of the price in return.
Two settings come attached. Time in force decides how long the order waits. A day order expires at the end of the session, a good-till-cancelled order rests on the book until it fills or you pull it, subject to whatever maximum your broker applies. Partial fills are normal rather than a fault: if only part of the size is available at your price, that part trades and the remainder keeps waiting. Resting orders also sit visibly on the book, which is one reason large ones are usually broken into pieces.
The mistake is hearing a guaranteed price and reading it as a guaranteed trade. A buy limit set slightly under the market can sit untouched through the whole move it was meant to catch, and a sell limit above the market does nothing while a price falls away beneath it. The surprise in the other direction is a pleasant one: a fill can come in better than the limit you set, because a limit is a boundary rather than a request.