A space buoy or marker buoy was a navigational marker placed in a particular location in space as an aide to travelers.
Upon encountering a First Federation warning buoy in 2266, Spock theorized that it might be a space buoy of some kind or potentially a form of "flypaper" to detain approaching starships. (TOS: "The Corbomite Maneuver")
Buoys could also be used as sensor devices in a way similar to mobile probes. In 2370, as a security measure, the Federation established a series of checkpoints and sensor buoys along their border with Cardassian space. (TNG: "Preemptive Strike")