Octave
Twelve semitones: the same note at another pitch. It is so consonant it is often mistaken for a unison; the difference is the brightness of the second note.
12 semitones Symbol P8 Perfect consonance
Listen
See
C C
E
B P8
G
D 0
A
E
0 1234567
C → C
Where you have heard it
Ascending
Somewhere Over the Rainbow the first two notes
The Christmas Song opening
Descending
Willow Weep for Me first leap
What it gets confused with
Frequently asked
How many semitones is an octave?
Twelve — the same note name, one register higher.
How do I recognise it by ear?
It is the first two notes of "Somewhere Over the Rainbow" and the opening of "The Christmas Song".
Why do the two notes sound like the same note?
Because the upper note vibrates at exactly twice the frequency of the lower one. The ear reads that ratio as sameness with a change of brightness.
There is an exercise track dedicated to recognising this by ear.
Train this