There are a number of names emerging and none of them is really simple and short. “Self-balancing Electric Unicycle” is the full name and this could be seen as the formally correct title. But an abbreviation based on the Chinese name for these single-wheel vehicles, which is 电动 独轮车 is becoming the norm. It translates directly to Electric Unicycle (电动=electric 独轮车=unicycle).
Abbreviations are sometimes EU or EUC
Other names we have seen are “Electric single-wheel scooter”, enthusiasts call it simply the “wheel”, British resellers seem to love the completely non-descript term “Unit”, and then there’s the umbrella term PEV (Personal Electric Vehicle) that also includes Electric Unicycles and many more vehicles.