Are there clear and commonly used names for these definitions?
I don't think there are completely definitive terms for the various usages, but the following is usage I have seen:
- Church mode for Medieval chant
- major key / minor key for common practice period, the "major/minor system" using tonic/dominant harmony. the other usage is a kind of generic diatonic, usually pop music, where the key indicates the tonic chord, but the harmony isn't necessarily tonic/dominant. These two usages might seem confusing, but the context is usually clear. If someone says a U2 song is in
A major, I expect
A major to be the tonic chord, etc. etc. but I don't necessarily expect tonic/dominant cadences and such. It isn't too hard to wrap your head around two different harmonic styles used to define a tonic chord and calling that "in the key of..."
- "color of", "flavor of" phrygian mode, etc. in jazz/rock/folk, but this usage is in classical style too with terms like phrygian cadence, or saying the Neapolitan chord has a phrygian feel, basically it's the major/minor system with certain altered scale degrees having modal color.
- By number, ex. Mode III is phrygian, Medieval chant
- "mode of a scale" seems to be a modern concept, jazz/rock/etc. like 'the altered scale is the seventh mode of melodic minor ascending.'
- this "mode" for that chord is a jazz concept, the usage treats mode and scale as basically synonymous and it's always that a mode/scale is some kind of embellishment of a chord and not a tonality
That list certainly isn't comprehensive, but I think it gives the sense of context for usage in many cases.
The case that seem "problematic" to me is pop music with alternating tonal centers and indefinite formal structure. All the tones to give a key signature will be present, many of the chord progressions will be conventional, a clear tonic will be present, but a tonic/dominant progression will be absent. Often the melodies are pentatonic or in some way don't use a leading tone. The music is groove based, often repetitious, in recordings there is often a fade out implying there is no formal ending. In a certain sense to define a "key", really a tonic, for such songs, is to imply an end point, and that is antithetical to the style. The problem is not really about the definition of "key", it more a matter of form and a infinitely repeating structure.
Using the song Live Is Life I suppose you could say it's in the "key of zero sharps/flats." From that we know what the palette of tones and diatonic chords is, but the tonic remains indefinite. It's a clumsy, unsatisfying wording, but I've never heard any standard wording. Usually it's music theory wonky argument over C major, or G mixolydian, A aeolian, D dorian, etc. that misses the whole point of the indefinite structure. Or, better yet, just say what it does: alternates between
G major and
A minor. Isn't that perfectly descriptive?
Below are a few readings on the concept of "key" that grabbed my attention in the past:
From Gjerdingen, Music in the Galant Style
Part of the difficulty resides in the word "key." By the second half the century—Clementi's time—the meaning of "key" was approaching its modem sense, as in "the key of Bt> major." In the first half of the century, "key" could also imply a note in a scale that received some temporary focus, as in "G—the sixth key in the hexachord on Bb." If we examine a repertory from the first half of the century, we ought to find more of the practices that were not yet "obliterated."
I mistakenly thought the following was said by Bartok, but it's from Halsey Stevens, The Life and Music of Bela Bartok, analyzing Bartok's music:
tonalities...are handled so freely that one is justified only in saying that they are "on"--not "in" this or that tonality....by this it is understood that these keynotes serve as orientation point: that the music is organized around them, modally or chromatically, freely fluctuating, using the keynotes as points of departure and points of repose...
Atcherson, Key and Mode in Seventeenth-Century Music
There isn't a particular quote that stuck with me. It's just twenty pages of detailed discussion of key and mode. He gives a nice bulleted list of "modes vs. keys" with items like: ambitus is an important aspect of mode definition and identification, but irrelevant to the concept of key. He really tries to explain the difference in technical terms. Of course "mode" in this case means mode like Gregorian chant, not the jazz usage of mode.