It's not unusual to see dyads described as intervals. Technically they're different. But where "triad" is used all the time, "dyad" just isn't used much.

Intervals are basically the number of semitones between two pitches. Life would be easy if you could just say "seven semitones", but in the context of scales and keys the intervals have names - second, third, etc - with modifiers that are somewhat context dependent.

Example: an augmented fourth and a diminished fifth are both six semitones wide, but you'd use one name or the other depending on the key/scale and other details.

Intervals that span more than an octave are usually called [number of octaves] + [usual name].