A microprobe was a type of small probe carried aboard Federation starships that was used in cases where conventional probes were too large to be effective.
While inside a nucleogenic cloud being in 2371, USS Voyager launched a class 4 microprobe. (VOY: "The Cloud")
Later that year, Voyager launched a microprobe into a micro-wormhole to trace its subspace bearing, but the probe became lodged in a gravitational eddy inside. Harry Kim subsequently modified the communication system so the probe could act as a subspace relay station, allowing Voyager to make contact with the Talvath on the other side. The probe was eventually crushed by the wormhole's collapse. (VOY: "Eye of the Needle")
In omitted dialogue from the final draft script of ENT: "The Aenar", Commander Charles Tucker III asked Commander T'Pol to hand him a (hand-held) microprobe, while they were working on assembling a telepresence unit, and T'Pol proceeded to do so.
According to the Star Trek: Deep Space Nine Technical Manual, some of the microprobes might also be called subminiature class 1-S sensor probes.