Nassi can be translated as President/Prince/Leader. An amalgam of the three English terms probably best communicates its intent.

Hillel II was preceded as Nassi by Judah II, and was succeeded as Nassi by Gamliel V.

Hillel held the position of Head of the Sanhedrin between 320–385 CE.

The Sanhedrin was the supreme rabbinical assembly, which had different morphings and levels–of–authority
over the ages. When Judea/Israel was autonomous, the Sanhedrin would tend to have (far greater) judicial and political power. When Judea/Israel was under foreign domination, it had consequently generally dramatically less judicial and political power, if any. For instance, under Rome in 33 CE, when Jesus was executed by Pontius Pilate, it had no substantive judicial power whatsoever. Its halachic power (power of adjudicating Jewish religious doctrine and related) over the centuries depended on many factors, including primarily its prestige at the moment.

In 33 CE, again, as an example, at the time of the crucifixion of Jesus, the Sanhedrin’s authority and influence was at a nadir, as one of the key players in the related Jewish religious hierarchy, Caiaphas the High Priest, was a co–opted appointee of Rome, instead of by the Jewish scholarly/religious elite. The Sanhedrin
was consequently delegitimized in Jewish eyes,
undermining its prestige and remaining authority.

361–363 CE: ROMAN EMPEROR JULIAN

–allows the Jews to return to Jerusalem (361–363 CE)