Here is as good of a definition that I have seen and it's rooted in the real-world of music and how music is read and played:
"The two basic dynamic indications in music are:
p or piano, meaning "soft", and[2][3]
f or forte, meaning "loud."[2][4]
More subtle degrees of loudness or softness are indicated by:
mp, standing for mezzo-piano, meaning "moderately soft", and
mf, standing for mezzo-forte, meaning "moderately loud".[5]
Beyond f and p, there are also
pp, standing for "pianissimo" and meaning "very soft", and
ff, standing for "fortissimo" and meaning "very loud".[5]
To indicate an even softer dynamic than pianissimo, ppp is marked, with the reading pianissimo possibile ("softest possible"). The same is done on the loud side of the scale, with fff being fortissimo possibile ("loudest possible").[6][7]
Note Velocity is a MIDI measurement of the speed with which the key goes from its rest position to completely depressed, with 127, the largest value in a 7-bit number, being instantaneous, and meaning as loud as possible.
Few pieces contain dynamic designations with more than three f's or p's. In Holst's The Planets, ffff occurs twice in Mars and once in Uranus often punctuated by organ and fff occurs several times throughout the work. It also appears in Heitor Villa-Lobos' Bachianas Brasileiras No. 4 (Prelude). The Norman Dello Joio Suite for Piano ends with a crescendo to a ffff, and Tchaikovsky indicated a bassoon solo pppppp in his Pathétique Symphony and ffff in passages of his 1812 Overture and the 2nd movement of his Fifth Symphony. Igor Stravinsky used ffff at the end of the finale of the Firebird Suite. ffff is also found in a prelude by Rachmaninoff, op.3-2. Shostakovich even went as loud as fffff in his fourth symphony. Gustav Mahler, in the third movement of his Seventh Symphony, gives the celli and basses a marking of fffff, along with a footnote directing 'pluck so hard that the strings hit the wood.' On another extreme, Carl Nielsen, in the second movement of his Symphony No. 5, marked a passage for woodwinds a diminuendo to ppppp. Another more extreme dynamic is in György Ligeti's Études No. 13 (Devil's Staircase), which has at one point a ffffff and progresses to a ffffffff. In Ligeti's Études No. 9, he uses pppppppp. In the baritone passage Era la notte from his opera Otello, Verdi uses pppp. Steane (1971) and others suggest that such markings are in reality a strong reminder to less than subtle singers to at least sing softly rather than an instruction to the singer actually to attempt a pppp. Usually, the extra f's or 'ps written reinforce either ff or pp, and are usually only for dramatic effect.
In music for marching band, passages louder than fff are sometimes colloquially referred to by descriptive terms such as "blastissimo".
Dynamic indications are relative, not absolute. mp does not indicate an exact level of volume, it merely indicates that music in a passage so marked should be a little louder than p and a little quieter than mf. Interpretations of dynamic levels are left mostly to the performer; in the Barber Piano Nocturne, a phrase beginning pp is followed by a diminuendo leading to a mp marking. Another instance of performer's discretion in this piece occurs when the left hand is shown to crescendo to a f, and then immediately after marked p while the right hand plays the melody f. It has been speculated that this is used simply to remind the performer to keep the melody louder than the harmonic line in the left hand. In some music notation programs, there are default MIDI key velocity values associated with these indications, but more sophisticated programs allow users to change these as needed."
Based on the above information, I don't know how it can be disputed that microdynamics and macrodynamics exist in the real world. And I say that knowing that anything and everything in the audio world can and will be debated until the dead horse is down to tooth, hair, and bone.
