A transitofMarsacross theSunas seen fromUranus takes place when the planet Mars passes directly between the Sun and Uranus, obscuring a small part of the Sun's disc for an observer on Uranus. During a transit, Mars can be seen from Uranus as a small black disc moving across the face of the Sun.
Naturally, no one has ever seen a transit of Mars from Uranus, nor is this likely to happen in the foreseeable future. Nevertheless, the next one will take place on 13 December 2018.
A transit could be observed from the surface of one of Uranus' moons rather than from Uranus itself. The times and circumstances of the transits would naturally be slightly different.