The Octave Interval (P8)

An interval is the distance between two notes, counted in semitones. Each of the thirteen simple intervals has its own page: reference songs that open with it, where it sits on the guitar fretboard, its inversion, and the chords built from it.

The Unison (P1) — 0 semitones
The Minor 2nd (m2) — 1 semitone
The Major 2nd (M2) — 2 semitones
The Minor 3rd (m3) — 3 semitones
The Major 3rd (M3) — 4 semitones
The Perfect 4th (P4) — 5 semitones
The Tritone (TT) — 6 semitones
The Perfect 5th (P5) — 7 semitones
The Minor 6th (m6) — 8 semitones
The Major 6th (M6) — 9 semitones
The Minor 7th (m7) — 10 semitones
The Major 7th (M7) — 11 semitones
The Octave (P8) — 12 semitones

The octave spans 12 semitones — the same note, higher — complete agreement. Measured up from a major-key tonic it lands on the octave, so it is diatonic to every major key. Inverted (the lower note moved up an octave) it becomes the unison.

The classic way to internalize it: sing a song that opens with it — Somewhere Over the Rainbow (the leap on "Some-where"). Then drill it on the ear trainer until you name it without thinking.