The word “interval” refers to the distance between two notes. The basic intervals are: Unison, 2nd, 3rd, 4th, 5th, 6th, 7th, and octave.
Each interval can be raised or lowered. We can divide the intervals into two groups:
Group 1: Unison, 4th, 5th, and octave
Group 2: 2nd, 3rd, 6th, and 7th.
Intervals in the first group are called “perfect.” Raising them by a half-step results in an “augmented” interval (for example, an augmented fourth). Lowering them by a half-step results in a “diminished” interval (for example, a diminished fifth).
The second group is the “major/minor” group. These intervals as referred to as “major” when they are untouched (Ex: a major third). If they are lowered by a half-step (aka “flatted”) then they are called minor intervals (Ex: a minor 6th).
Here’s the basic intervals and their distance:
Diminished or flat 2nd
1 whole-step + 1 half-step
2 whole steps
2 whole steps + 1 half-step
Augmented 4th/ Diminished 5th
3 whole steps
3 whole steps + 1 half-step
4 whole steps
4 whole steps + 1 half-step
5 whole steps
5 whole steps + 1 half-step
6 whole steps
Here are two examples of how each sounds, starting from a unison and going up by half-steps to an octave:
Melodically (notes played individually, back-to-back):
Harmonically (notes played simultaneously):
Side note: You will see varying notation and terminology for intervals. A diminished fifth can be written as “D5.” Guitarists tend to call a diminished fifth a “flat-five.” Just something to keep in mind.
We can also divide them into two different groups based on their consonant and dissonant nature:
Consonant intervals: unison, octave, all 3rds, some 4ths, 5ths, and all 6ths. (Don’t worry about the “some 4ths.” We’ll get to this later).
Dissonant intervals: all 2nds, some 4ths, all 7ths, and all augmented/diminished intervals.
One last note: It can be beneficial to think of a certain interval as a compound of two other intervals. For example, I think of a 5th as adding a minor 3rd and major 3rd. A 6th is a 5th with an added 2nd, etc. These are just cognitive tricks that will come with practice.
TL;DR: An interval is the distance between two notes. The basic intervals are: unison, 2nd, 3rd, 4th, 5th, 6th, 7th, octave. The perfect intervals are: Unison, 4th, 5th and octave. The major/minor intervals are: 2nd, 3rd, 6th, and 7th. The consonant intervals are: unison, all 3rds, some 4ths, 5ths, and all 6ths. The dissonant intervals are: all 2nds, some 4ths, all 7ths, and all augmented/dimin