I'm just going to give my two cents even if it is contrary to the already given answers.
But I always thought that two notes aren't a chord, they are an
interval. And according to the Intervals Wikipedia page, an interval
These terms are somewhat related. An interval represents the space between two sets of notes either harmonic or melodic. The notes of chords are a set interval from each other (a third.)
The thing is that the main tonal center for the character of a chord is the third. The fifth is a lot less important and only gives any meaningful function when you are busy with augmented or diminished chords.
So in essence if you have a note and a third you have a rudimentary chord. You also often see in tonal harmony seventh chords that only have three chord notes with the fifth omitted.
It is in essence a case of all chords consist of intervals but not all intervals are in chords.
the difference between two pitches.
I would phrase it the distance between two pitches.
...any harmonic set of two or more notes that is heard as if sounding
I don't think this is entirely correct. You have both arpeggios and broken chords that are not played at the same time and to me still count as chords in the broader sense of the word.
Also piano players often have to roll there chords and if this is the case the notes of the chords are not played exactly at the same time.