What does shi'a mean?

Answer 1

Shia means group or party of followers.

This word is mentioned in the Quran many times.

"And most surely Abraham was among the Shia of him (i.e., Noah)" (Quran 37:83)

The Shia used in reference to the group of Muslims is shortened for Shia' t' Ali which means "party of Ali (as)"
Follower [of Ali]

Answer 2

The word Shia (Arabic: شيعة‎ shīʻah /ˈʃiːʕa/) means follower[8] and is the short form of the historic phrase shīʻatu ʻAlī (شيعة علي /ˈʃiːʕatu ˈʕaliː/), meaning "followers of Ali", "faction of Ali", or "party of Muhammad's son-in-law and cousin Ali, whom the Shia believe to be Muhammad's successor in the Caliphate. Twelver Shia (Ithnā'ashariyyah) is the largest branch of Shia Islam and the term Shia Muslim is often taken to refer to Twelvers by default.

The Shia represent the second largest denomination of Islam and adherents of Shia Islam are called Shias or the Shi'a as a collective or Shi'i individually