Meaning of U3 in English

n. small portable USB drive developed so that applications can be performed directly from a specially designed USB flash drive without leaving a bit of data on the host computer (can be used for storing pictures, music and documents; enables to store and run software programs; emails and calendars can be carried in it as well)

Explanatory English dictionary bed edition.      Толковый словарь английского языка Редакция bed.