Although there are only seven different letters in the musical alphabet, it is possible to find higher and lower versions of them at various places on the violin. The distance between any note and its next higher or lower version is called an octave.
You already know two A notes – the open second string and the higher version on the first string (E string). These two A notes are an octave apart.
The first and last notes of a major scale always have the same name. In the A major scale the distance from the lowest A to the A note above it is one octave (8 notes).