The embouchure is the use of facial muscles and the shaping of the lips to the mouthpiece of woodwind instruments or the mouthpiece of the brass instruments.
The word is of French origin and is related to the root bouche (fr.), 'mouth'.
The proper embouchure allows the instrumentalist to play the instrument at its full range with a full, clear tone and without strain or damage to one's muscles.
Read more about Embouchure: Brass Embouchure, Polyphonic Embouchure (finger-embouchure)