A definite series of chords, forming a passage with some harmonic unity or dramatic meaning. Chords in progressions are labeled with Roman numerals (I, II, etc.) while scale degrees, and upper structures (q.v.), are labeled with Arabic numerals (1, 2, etc.).
Chord progressions evolved from the study of contrapuntal music. (The most widely used examples of this would be the Chorales written by JS Bach.) In analyzing the Chorales, music theorists realized that music functioned vertically (chordal harmony) as well as horizontally (contrapuntally). It was recognized that the notes of the scale demonstrated either an active quality or resting quality. Active tones predicted their movement in a specific direction, whereas resting tones could move up, down, or be content to stay put. Just as the notes in the diatonic scale had a tendency to predict their movement, the chords derived from the four contrapuntal lines of the chorale followed same characteristics exhibited by the notes of the scale. The active or resting quality and direction a chord tends to move in is determined by the quality of the notes that comprise it. A chord with one active tone and two resting tones will be considered a resting chord, however, the active tone can be used to determine its direction. Conversely, a chord with more active tones than resting will tend to move in a specific direction. Keep in mind that the active or resting quality of a note or chord is dependent on its relationship to other notes in the scale. E.g., in the key of C Major, scale degree three is the note E which is a resting tone. In the key of F Major, the note E becomes scale degree seven, which is an active tone. Because of these and a number of other relationships that go beyond the scope of this definition, our ear tells us that there is a special relationship between scale degree one scale, degree four, and scale degree five. The relationship between five and one is extremely important since it determines the tonal center of the key. A triad on scale degree five always wants to resolve to scale degree one. A triad built on scale degree one can act like scale degree five in relation to four, with the four-chord acting like scale degree one. What this means is that in the key of C Major for example, a chord built on scale degree one (C major) moves easily to a triad built on scale degree four (F Major) which can move to scale degree five’s triad, (G major), which in turn desperately wants to go back to scale degree one. Basically we’ve just described the most common progression in popular music, the Blues Progression: I, IV, V.