Ionian mode is a musical mode or, in modern usage, a diatonic scale also called the major scale.
It is the name assigned by Heinrich Glarean in 1547 to his new authentic mode on C (mode 11 in his numbering scheme), which uses the diatonic octave species from C to the C an octave higher, divided at G (as its dominant, reciting tone/reciting note or tenor) into a fourth species of perfect fifth (tone–tone–semitone–tone) plus a third species of perfect fourth (tone–tone–semitone): C D E F G + G A B C.  This octave species is essentially the same as the major mode of tonal music. 
Church music had been explained by theorists as being organised in eight musical modes: the scales on D, E, F, and G in the "greater perfect system" of "musica recta,"  each with their authentic and plagal counterparts.
Glarean's twelfth mode was the plagal version of the Ionian mode, called Hypoionian (under Ionian), based on the same relative scale, but with the major third as its tenor, and having a melodic range from a perfect fourth below the tonic, to a perfect fifth above it. 
In music theory, a diatonic scale is any heptatonic scale that includes five whole steps and two half steps (semitones) in each octave, in which the two half steps are separated from each other by either two or three whole steps, depending on their position in the scale. This pattern ensures that, in a diatonic scale spanning more than one octave, all the half steps are maximally separated from each other.
In music theory, the term mode or modus is used in a number of distinct senses, depending on context.
The major scale is one of the most commonly used musical scales, especially in Western music. It is one of the diatonic scales. Like many musical scales, it is made up of seven notes: the eighth duplicates the first at double its frequency so that it is called a higher octave of the same note.
In music, the subtonic is the degree of a musical scale which is a whole step below the tonic note. In a major key, it is a lowered, or flattened, seventh scale degree. It appears as the seventh scale degree in the natural minor and descending melodic minor scales but not in the major scale. In major keys, the subtonic sometimes appears in borrowed chords. In the movable do solfège system, the subtonic note is sung as te.
Dorian mode or Doric mode can refer to three very different but interrelated subjects: one of the Ancient Greek harmoniai ; one of the medieval musical modes; or—most commonly—one of the modern modal diatonic scales, corresponding to the piano keyboard's white notes from D to D, or any transposition of itself.
Mixolydian mode may refer to one of three things: the name applied to one of the ancient Greek harmoniai or tonoi, based on a particular octave species or scale; one of the medieval church modes; or a modern musical mode or diatonic scale, related to the medieval mode.
The Aeolian mode is a musical mode or, in modern usage, a diatonic scale also called the natural minor scale. On the white piano keys, it is the scale that starts with A. Its ascending interval form consists of a key note, whole step, half step, whole step, whole step, half step, whole step, whole step. That means that, in A aeolian, you would play A, move up a whole step to B, move up a half step to C, then up a whole step to D, a whole step to E, a half step to F, a whole step to G, and a final whole step to a high A.
The Phrygian mode can refer to three different musical modes: the ancient Greek tonos or harmonia sometimes called Phrygian, formed on a particular set of octave species or scales; the Medieval Phrygian mode, and the modern conception of the Phrygian mode as a diatonic scale, based on the latter.
The modern Lydian mode is a seven-tone musical scale formed from a rising pattern of pitches comprising three whole tones, a semitone, two more whole tones, and a final semitone.
Heinrich Glarean was a Swiss music theorist, poet and humanist. He was born in Mollis and died in Freiburg.
In the musical system of ancient Greece, genus is a term used to describe certain classes of intonations of the two movable notes within a tetrachord. The tetrachordal system was inherited by the Latin medieval theory of scales and by the modal theory of Byzantine music; it may have been one source of the later theory of the jins of Arabic music. In addition, Aristoxenus calls some patterns of rhythm "genera".
The Locrian mode is either a musical mode or simply a diatonic scale. On the piano, it is the scale that starts with B and only uses the white keys from there. Its ascending form consists of the key note, a half step, two whole steps, a further half step, and three more whole steps.
A heptatonic scale is a musical scale that has seven pitches, or tones, per octave. Examples include the major scale or minor scale; e.g., in C major: C D E F G A B C—and in the relative minor, A minor, natural minor: A B C D E F G A; the melodic minor scale, A B C D E F♯G♯A ascending, A G F E D C B A descending; the harmonic minor scale, A B C D E F G♯A; and a scale variously known as the Byzantine, and Hungarian, scale, C D E♭ F♯ G A♭ B C. Indian classical theory postulates seventy-two seven-tone scale types, collectively called thaat, whereas others postulate twelve or ten seven-tone scale types.
The Hypolydian mode, literally meaning "below Lydian", is the common name for the sixth of the eight church modes of medieval music theory. The name is taken from Ptolemy of Alexandria's term for one of his seven tonoi, or transposition keys. This mode is the plagal counterpart of the authentic fifth mode.
The Hypophrygian mode, literally meaning "below Phrygian ", is a musical mode or diatonic scale in medieval chant theory, the fourth mode of church music. This mode is the plagal counterpart of the authentic third mode, which was called Phrygian. In the Middle Ages and Renaissance this mode was described in two ways: the diatonic scale from B to B an octave above, divided at the mode final E ; and as a mode with final E and ambitus from the A below to the C above. The note A above the final had an important melodic function. The melodic range of the ecclesiastical Hypophrygian mode therefore goes from the perfect fourth or fifth below the tonic to the perfect fifth or minor sixth above.
The Hypodorian mode, a musical term literally meaning 'below Dorian', derives its name from a tonos or octave species of ancient Greece which, in its diatonic genus, is built from a tetrachord consisting of a semitone followed by two whole tones. The rising scale for the octave is a single tone followed by two conjoint tetrachords of this type. This is roughly the same as playing all the white notes of a piano from A to A: A | B C D E | (E) F G A. Although this scale in medieval theory was employed in Dorian and Hypodorian, from the mid-sixteenth century and in modern music theory they came to be known as the Aeolian and Hypoaeolian modes.
Ambitus is a Latin term literally meaning enclos[ur]e, and in Medieval Latin means the "range" of a melodic line, most usually referring to the range of scale degrees attributed to a given mode, particularly in Gregorian chant. In Gregorian chant specifically, the ambitus is the range, or the distance between the highest and lowest note. Different chants vary widely in their ambitus. Even relatively florid chants like Alleluias may have a narrow ambitus. Earlier writers termed the modal ambitus "perfect" when it was a ninth or tenth, but from the late fifteenth century onward "perfect ambitus" usually meant one octave, and the ambitus was called "imperfect" when it was less, and "pluperfect" when it was more than an octave.
A Gregorian mode is one of the eight systems of pitch organization used in Gregorian chant.
The Hypoionian mode, literally meaning "below Ionian", is the name assigned by Henricus Glareanus in his Dodecachordon (1547) to the plagal mode on C, which uses the diatonic octave species from G to the G an octave higher, divided at its final, C. This is roughly the same as playing all the white notes of a piano from G to G: G A B C | (C) D E F G.
The Hypoaeolian mode, literally meaning "below Aeolian", is the name assigned by Henricus Glareanus in his Dodecachordon (1547) to the musical plagal mode on A, which uses the diatonic octave species from E to the E an octave above, divided by the final into a second-species fourth (semitone–tone–tone) plus a first-species fifth (tone–semitone–tone–tone): E F G A + A B C D E. The tenor or reciting tone is C, mediant B, the participants are the low and high Es, the conceded modulations are G and D, and the absolute initials are E, G, A, B, and C.