A promontory is a high, rocky cliff jutting into a body of water. A promontory is just the kind of thing a heroine will threaten to throw herself off of if the love of her life does not return to her.
A promontory can be a foreland, headland, or rocky cliff. Think Ireland’s Cliffs of Moher jutting into the sea. Connect promontory with prominent, "important or sticking out," project, "estimate forward" and protrude, "stick out," which also carry this sense of jutting out. In anatomy, promontory can refer to a projecting part of the body.