But the high priest rose up, and all who were with him (that is, the party of the Sadducees), and filled with jealousy, they arrested the apostles and put them in the public prison.  (Acts 5:17-18)

The word ‘party’ translates the Greek noun αἵρεσις (hairesis) and stands for people who strongly self-identify with each other (the verb hairéomai means ‘to personally select’), in other words to cohere as a sect or a religious or philosophical group.

The Romano-Jewish historian, Flavius Josephus, writing in the second half of the first century AD, identified four distinct groupings of Jews.  For ‘party’, it is best not to consider them in modern terms like political parties, or even organisations (with rules and membership) rather imagine a cross between a faction and a coterie or socio-religious affiliations, although ‘sect’ can certainly be applied in one instance.