From The Art and Popular Culture Encyclopedia
In music theory, the term key is used in many different and sometimes contradictory ways. A common use is to speak of music as being "in" a specific key, such as in the key of C major or in the key of F-sharp. Sometimes the terms "major" or "minor" are appended, as in the key of A minor or in the key of B-flat major. Although the concept of musical key can be a complicated subject when examined closely, broadly speaking the phrase in key of C means that C is music's harmonic center or tonic. Note that the letter-name "C" does not indicate a single specific pitch but rather all pitches with the letter name C (sometimes called a pitch class). The terms "major" and "minor" further imply the use of a major scale or a minor scale. Thus the phrase in the key of E major implies a piece of tonal music harmonically centered on the note E and making use of a major scale whose first note, or tonic, is E. Although the term "key" is commonly used this way, actual music can rarely be described so simply. This overview of the term also makes many assumptions and may not hold true for all forms of music.