The term astronomical marker referred to astronomical phenomena useful for determining one's navigation.
In 2372, the Cardassian-originated missile Cardassian ATR-4107 used astronomical markers to verify it's current position. From this, it concluded that it was in the Delta Quadrant, though it then discarded the conclusion in light of that being highly unlikely to be true. (VOY: "Dreadnought")
In 2373, the Nyrian Dammar upon mysteriously arriving on USS Voyager was requested to look at a star chart to see if he could recognize any astronomical markers offering a clue as to where his system of origin was in relation to the ship. (VOY: "Displaced")