Picture a people in trouble who have been promised that one person is coming. He has been chosen — in the old picture, oil was poured on his head to mark him out — and when he arrives he will lift the whole people out of their trouble. You can think of messiah as that picture: one chosen person, a promise, and a crowd that is waiting. Everything the word does later keeps some part of that: who the person is, which tradition is waiting, and how seriously the waiting is meant.
The word goes back to Biblical Hebrew māšîaḥ, meaning 'anointed' — marked out with oil as a king or priest was. It travelled through Aramaic məšīḥā, Ancient Greek Messías and Latin Messīās into Middle English Messyas, and then into English. The old sense of 'the one who has been chosen and marked' is still what holds the modern uses together.
這個字可以追到聖經希伯來文 māšîaḥ，意思是「受膏者」——像立王或立祭司那樣被抹油做記號的人。它經過亞蘭文 məšīḥā、古希臘文 Messías、拉丁文 Messīās，再從中古英文 Messyas 進到英文。「被選定、被做記號的那一位」這個老意思，正好是今天各種用法共同的骨架。
Wiktionary entry 'messiah' supplied in this packet (source_ref kaikki:3f6f69a190036ae4:324669827): Middle English Messyas < Latin Messīās < Ancient Greek Μεσσίας < Aramaic ܡܫܺܝܚܳܐ (məšīḥā) < Biblical Hebrew מָשִׁיחַ (māšîaḥ, 'anointed').
Three branches
神所差來的救世主
這一組保留了完整的畫面：被神指定的那一位，以及一群在等他的人。兩個意思的差別不在字義，而在說話的人相信他來了沒有。
noun · B2
In Jewish belief, the great leader whom God will send in the future to save the world and bring peace. In this meaning the word is written the Messiah, with a capital letter, and the person has not yet come.
彌賽亞（猶太教等待的救世主）
借到其他傳統與故事裡
畫面沒變，只是換了一個傳統：一樣有預言，一樣有人在等一位要來帶大家得自由的人。這是意思上的延伸，不是說歷史上真的從那裡分出去。
noun · C1
An awaited saviour in a religion other than Judaism and Christianity, or in a story: a leader that people believe has been promised and will come to set them free.
（其他宗教或故事裡的）救世主
日常的救星
宗教的部分脫落了，只留下「只要他一來，一切就有救」的那份期待。所以這個用法小寫，而且常常帶點誇張或調侃。
noun · B2
Someone that a group of people believe will solve all their problems and put everything right.
救星