Messiah (mâšîah, "the anointed one"): Jewish religious concept, a future savior who will, in some sense, come to restore Israel. Both the nature of the Messiah and the restoration were matters of debate.
Roots of the concept: anointment
The word Messiah renders the Aramaic word mešîhâ', which in turn renders the Hebrew mâšîah. In Antiquity, these words were usually translated into Greek as Christos and into Latin as Christus, whence the English word Christ. All these words mean simply "anointed one", anointment being a way to show that a Jewish leader had received God's personal help. Take, for example, the anointment of the high priest:
Bring Aaron and his sons to the Tent of Meeting and wash them with water. Take the garments and dress Aaron with the tunic, the robe of the ephod, the ephod itself and the breastpiece. Fasten the ephod on him by its skillfully woven waistband. Put the turban on his head and attach the sacred diadem to the turban. Take the anointing oil and anoint him by pouring it on his head.note[Exodus 29.4-8.]
Then they acknowledged Solomon son of David as king [...], anointing him before the Lord to be ruler and Zadok to be priest.note[1 Chronicles 29.22b.]
This was a very common custom in the ancient Near East. In Babylonia and Assyria, sesame oil was symbolically poured over the heads of brides, people involved in certain property transactions, and freed slaves. Anointed priests (pašîšû) were an important class of the Babylonian clergy. In the Epic of Gilgameš, in the story of the Great Flood, even the Ark is annointed.note[XI.65ff.] Another person who had to be anointed, was the king, but this seems to have been a custom of the Jews only.
After they had come down from the high place to the town, Samuel talked with Saul on the roof of his house. They rose about daybreak and Samuel called to Saul on the roof, "Get ready, and I will send you on your way." When Saul got ready, he and Samuel went outside together. As they were going down to the edge of the town, Samuel said to Saul, "Tell the servant to go on ahead of us," -and the servant did so- "but you stay here awhile, so that I may give you a message from God." Then Samuel took a flask of oil and poured it on Saul's head and kissed him, saying, "Has not the Lord anointed you leader over his inheritance?"note[1 Samuel 9.25-10.1.]
Jesse made seven of his sons pass before Samuel, but Samuel said to him, "The Lord has not chosen these." So he asked Jesse, "Are these all the sons you have?" "There is still the youngest," Jesse answered, "but he is tending the sheep." Samuel said, "Send for him; we will not sit down until he arrives." So he sent and had him brought in. He was ruddy, with a fine appearance and handsome features. Then the Lord said, "Rise and anoint him. He is the one." So Samuel took the horn of oil and anointed him in the presence of his brothers, and from that day on the Spirit of the Lord came upon David in power.note[1 Samuel 16.10-13.]
One day the trees went out to anoint a tree for themselves...note[Judges 9.8; first line of a fable.]
Even prophets might be anointed:
The Lord said to Elijah: "Go back the way you came, and go to the Desert of Damascus. When you get there, anoint Hazael king over Aram [= Syria]. Also, anoint Jehu son of Nimshi king over Israel, and anoint Elisha son of Shaphat from Abel Meholah to succeed you as prophet."note[1 Kings 19.15-16.]
The Spirit of the Sovereign Lord is on me, because the Lord has anointed me to preach good news to the poor.note[Isaiah 61.1.]
It should be stressed that there was no clear-cut distinction between priests, kings and prophets. That would have been most unusual in Antiquity, because the ancients always regarded kingship as something religious. We must not be surprised, therefore, to read that the legendary king David acted as priest.note[E.g., 2 Samuel 6.12-19.]
Conversely, (high-)priests could behave like kings. This was less common and more or less specific for the priests of Jerusalem; it may go back to the royal priesthood of the Jebusites, the tribe living in Jerusalem before David made it his capital. The most famous example is Melchizedek, who once prepared supper for Abraham and gave him bread and wine.note[Genesis 14.18.] From 152 BCE on, the high-priests were the highest authorities and often combined priestly and royal authority. In the first century BCE, when most of our sources were written, they officially occupied both offices.
We can also read stories about kings acting as prophets. For example, David predicts the future in 2 Samuel 23.1-7.
Of course there was no need for clear-cut distinction between priests, kings and prophets. What mattered was that all these people were anointed and were considered to have God's special attention.