Diatonic Intervals are the notes that appear in the Major Scale. Diatonic Intervals can be Perfect or Major.
Diatonic notes are the notes in the Major scale. Diatonic intervals consist of notes that appear unaltered in the major scale.
Here's the C major scale written in two octaves. All these notes appear in the C major scale without alteration. They are all Diatonic Intervals to the note C. Diatonic intervals are either Perfect or Major.
The perfect intervals are Unison, 4th, 5th and octaves. All other diatonic notes are labeled as major. As notes repeat in higher octaves, perfect and major remain the same.