6. Definitions of terms

Twelve-tone equal temperament divides the octave into 12 semitones of 100 cents each.
Just intonation
Just intonation or pure intonation is any musical tuning in which the frequencies of notes are related by ratios of small whole numbers.
Middle C
The fourth C key from left on a standard 88-key piano keyboard is designated C4 (SPN) (Do3).
See Scientific Pitch Notation for numbering.
The property of a musical tone that is determined by its frequency
Pitch Class
For example, the pitch class “C” is the set {Cn : n is an integer} = {…, C−2, C−1, C0, C1, C2, C3 …}
The name of any interval is further qualified using the terms perfect (P), major (M), minor (m), augmented (A), and diminished (d).
A semitone, also called a half step or a half tone, is the smallest musical interval commonly used in Western tonal music. It is defined as the interval between two adjacent notes in a twelve-tone equal temperament.
Scientific Pitch Notation (SPN)
According to the Scientific Pitch Notation, A4 corresponds to a tone of frequency 440 Hz, note A and octave 4. A4 corresponds to La3 in Latin notation, where the octave bellow 1 is -1. The MIDI specification range from C-1 to G9.

a degree, grade, or rank in a scale

the interval between two adjacent notes in a musical scale