Short answers, pulled from the story.
A mode in music theory is a type of musical scale coupled with a set of characteristic melodic and harmonic behaviors. The term has a twofold sense, denoting either a particularized scale or a generalized tune, as proposed by Harold S. Powers in 2001. Outside Western classical music, the term also covers analogous systems such as maqam, raga, and pathet.
The seven modern diatonic modes are Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian, and Locrian. Each uses the same notes as the major scale but starts from a different scale degree, producing a distinct interval sequence. Ionian is identical to the major scale, and Aeolian is identical to the natural minor scale.
Aristotle wrote in the Politics, composed around 350 BC, that the Mixolydian moves listeners to grief and anxiety, relaxed harmoniai produce mellowness, the Dorian generates moderation and firmness, and the Phrygian creates ecstatic excitement. He concluded that music is capable of creating a particular quality of character in the soul and should be used in the education of the young.
The oldest medieval treatise regarding modes is Musica disciplina by Aurelian of Reome, dating from around 850. The earliest Western source actually using the system of eight modes is the Tonary of St Riquier, dated to between roughly 795 and 800. Hermannus Contractus was the first theorist to define modes as partitionings of the octave.
The Swiss theorist Henricus Glareanus added four modes to the traditional eight in his 1547 publication the Dodecachordon, including the Aeolian (mode 9), Hypoaeolian (mode 10), Ionian (mode 11), and Hypoionian (mode 12). Gioseffo Zarlino later adopted Glareanus's system in 1558 but revised the numbering in 1571 and 1573, producing two conflicting systems.
The Locrian mode is traditionally considered theoretical because the triad built on its first scale degree is diminished. Diminished triads are not consonant and cannot serve cadential endings or be tonicized according to traditional harmonic practice. It is the only one of the seven modern modes in which the chords built on the tonic and dominant scale degrees are separated by a diminished fifth rather than a perfect fifth.