Where does the word/name Emmanuel come from?

The word Emmanuel is the Hebrew for 'God is with us'.  For Christians the birth of Jesus is the physical incarnation of this:  
Mt:1:23: Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us. Luke:1:26-39: 26:
This is made explicit in  Hark the Herald Angel Sing: Glory to God in the ... Jesus our Emmanuel. ... 

It is also the the theme of the most beautiful carols: