Scales

A sequence of musical Intervals, conventionally progressing upwards from a Tonic note. From the Italian scala - ladder or staircase.

Church plainsong and Folk Song was / is based on Modes of which there were a number usually established as twelve. Two remain today in mainstream music and have been the principal scales for composition since 1600.These are the Major and Minor scales derived directly from the Ionian and Aeolian modes respectively. Other modes are still to be found in folk music, most frequently the Mixolydian and the Dorian.

In conventional Western music, there are 8 notes in a scale, making an Octave. Each note in the scale is given a name denoting it's importance, irrespective of what the starting note is.

The tonic is also called the root or key note.
The dominant is the next most important as it dominates the scale and is a pivotal note
The mediant is mid-way between tonic and dominant.
These three notes together form the triad which is the major or minor Chord depending on the Intervals in the scale.