In Arabic, the term “Shi'a” originally
means one, two or a group of followers. In the Glorious Qur’an, this
term is used several times in this sense. For example, God speaks of one
of the followers of Moses as one of his Shi'a in the verse (28:15).
Elsewhere, Abraham is introduced as a Shi'a of Noah (37:83).
In the beginning of the history of Islam, the term “Shi'a” was used in its original or literal sense for followers of different people. For example, some hadiths speak of the Shi'a of Ali ibn Abi Talib and others of the Shi'a of Mu‘awiyah ibn Abi Sufyan. However, gradually the term acquired a secondary or technical meaning, i.e. the followers of Ali, those who believed in his Imamat (divinely appointed leadership).
Ref: Shia.com (with minor changes)