In the name of Allah
Literally, it has two meanings, among: following an individual or a group.
Idiomatically, it is said/applied for the Muslims who believe (accept) the immediate Imamate/Caliphate of Ali (Imam Ali (a.s.)) and likewise believe:
Imams and the successor of Prophet Muhammad are determined (appointed) through Nass-al-Shar’i. And the Imamate of Imam Ali (a.s.) and other Shi’a Imams has proved by Nass-al-Shar’i too.
Note: Nass is deemed as a Fiqhi expression which is said to the Hokm (ruling) which is inferred from the Holy Quran apparent or Prophet and Imam’s hadiths.
According to Wikipedia:
Shia (/ˈʃiːə/; Arabic: شيعة Shīʿah, from Shīʻatu ʻAlī, followers of Ali, Persian: شیعه Shīye or Shīʿee) is a branch of Islam which
holds that the Islamic prophet Muhammad designated Ali ibn Abi Talib
as his successor (Caliph). Shia Islam primarily contrasts with Sunni
Islam, whose adherents believe that Muhammad did not appoint a
successor. Instead they consider Abu Bakr (who was appointed Caliph
through a Shura, i.e. consensus) to be the correct Caliph.