X marks the spot

This sign or mark (not necessarily an X) indicates the specific or exact location (of something). I drew an elaborate treasure map for my daughter's birthday, with X marking the spot where I've hidden her presents. If you look at this financial chart, X marks the spot across all of them where the company began to seriously falter.

This is the exact spot. (Sometimes the speaker will draw an X in the spot while saying this.) This is where the rock struck my car—X marks the spot. Now, please move that table over here. Yes, right here—X marks the spot.

This mark shows the location, as in On the postcard, X marks the spot where we picked blueberries. Although the use of a cross or X is probably much older, this term was first recorded in 1813.

This mark shows the scene of the crime, the hiding place of a treasure, or some other special location. Although this term dates only from the nineteenth century, the use of a cross or the letter X as a special indicator is surely much older. The OED’s earliest citation is from a letter by Maria Edgeworth in 1813: “The three crosses X mark the three places where we were let in.” The term often appeared in romantic pirate stories in which hidden treasure marked on a map figured, as in Robert Louis Stevenson’s Treasure Island (1883).