Why is Jesus in the Qur'an called "Al-Masih" namely, "The Messiah"?
No one else in the Qur'an is, or accordingly possibly could be, the Messiah. Jesus is not a messiah or one of the messiahs, he is Al-Masih - the Messiah. Also, what is most significant is that the title is applied solely to Jesus in the Qur'an and that its definitive quality is carefully defined by the use of the article - Al-Masih, namely, the Messiah. Indeed the title is never used in the Qur'an without the definite article. This rules out any possibility that the title can be applied to anyone else.
The Qur'an's acknowledgement that Jesus was indeed "The Messiah" comes as a surprise, for it denies that Jesus was anything more than a prophet, whereas the promises of God about the coming Messiah had made it plain that he would be far greater than just a prophet. The Christian confession that Jesus is the Lord and Saviour of all men is thus consistent with the teachings of the former prophets that the coming Messiah would be the supreme man of history, far above all the prophets (2 Samuel 7.12-14).
The Qur'an, on the other hand, declares Maal Masihubnu Maryama illa rasul - "The Messiah, son of Mary, was no more than an apostle" - like the other apostles who had passed away before him (Surah 5.78).
Why, then, does the Qur'an also acknowledge that Jesus was "The Messiah" if it denies that he was anything more than a prophet' It seems hard to avoid the conclusion that the Qur'an is contradicting itself here, especially when it offers no alternative explanation of the title.
In an argument with the Jews, Jesus, having answered all their questions, challenged them to identify the coming Messiah - whose son was he? They responded "the son of David" to which he replied:
How is it then that David, inspired by the Spirit, calls him Lord, saying, ‘The Lord said to my Lord, sit at my right hand till I put your enemies under your feet’? If David thus calls him Lord, how is he his son? Matthew 22.43-45
Explain the above, you said yourself, although I disagree witht the following comment - "The Jews also share the same meaning of the word Messiah!