It's somewhat subjective, but my problem is how does one name a dyad, apart from calling it an interval.
Thus power chord becomes D5 - an interval.
Most chords, to me, need the blend of minimum three notes.
Majors and minors will have that interval between root and three.
Some will argue, corectly to an extent, that actually the root will usually sound a 5th as its second harmonic, thus it's going to sound anyway - a good reason why some jazz chords leave out the P5.
However - if that's the case, is that actually a two note 'chord', as we hear and perceive three notes...
So, what do we call other two note 'chords'?
All the 'proper' chord names won't work, as they're specific to the notes in that chord.
Taking a note out of even a three note chord stops it being an xyz.
The power chord works sonically because the 3 is missing.
Favoured by guitarists using distortion/overdrive.
With those effects, the harmonics of each note are accentuated, and those belonging to the 3 clash with the others.
Root and 5 harmonics generally sound o.k. together, so it works.
But, to me at least, it still isn't a chord, but an interval - a dyad.
music.stackexchange.com/users/2808/Tim
A chord is in no way always played at the same time.
Arpeggios and broken-chords are chords in the broader sense of the word and neither are played at the same time.
A proper melody also has a harmonic structure built on chords and again none of the notes are played at the same time.
Also a chord can have three notes as maybe the stadard but it could also have four notes or even five, there is nothing forcing a chord to only have three notes, it can be any number of thirds.
As for the specifics about two note chords with guitar, that is much more a peculairity of the guitar as instrument.
The guitar has a wealth of harmonics on each string that make playing two note chords still work, but you are right if you take the concepts away from the guitar
then G - B would be the space between two notes ie an interval.
music.stackexchange.com/users/7306/Neil Meyer
Terminology is fickle.
If you want to be exact, don't try to find the "exact" line between what is considered a "chord" or not, because the line will always be arbitrary, as the other answers show.
Rather, specify the tones and don't worry about whether they constitute a "chord" or not.
music.stackexchange.com/users/26744/Scott Wallace
A chord is, simply put, a group of notes (usually three or more although 2 can still be considered a chord) played as a basis of harmony.
They do not necessarily have to be played simultaneously.
So yes 2 notes CAN be considered a chord but it depends on their intervals and their relationship with a melody.
Although it is a hard question to answer because 'what a chord is' can almost be subject to personal opinion.
music.stackexchange.com/users/39554/Ben Hughes
A chord is two or more notes played together.
An interval is the distance between 2 notes.
For example a 2 note chord might be the A5 power chord.
music.stackexchange.com/users/37653/Charlie Turnbull
The most basic chord to which we give a name is a triad, a three-note chord.
C major, D minor, Eb diminished etc.
Two notes, or even one note can suggest or imply a triad, but they cannot fully define it.
And that is what people mean when they argue that you need three notes for it to be a 'chord'.
In more general terminology, we often call two or more different notes a 'chord'.
music.stackexchange.com/users/17003/Laurence Payne
I was studying music, and in my book it says that two notes played simultaneously are considered an  Interval  while three notes are considered a  Chord .Following up this question  Why are two pitches considered a chord? .   Are dyads considered to be chords?  If yes, then can we actually define a chord with just two notes, for example if we have G and B,  which are suitable for both Em and GM.  If not, why are power chords considered chords when they only have two notes?
Are two pitches played simultaneously considered a chord?
music.stackexchange.com
<theory><chords><chord-theory>
music.stackexchange.com/questions/57911
music.stackexchange.com/users/40584/Jaafar Jumaa
The most basic chord to which we give a name is a triad, a three-note chord. C major, D minor, Eb diminished etc. Two notes, or even one note can suggest or imply a triad, but they cannot fully define it. In more general terminology, we often call two or more different notes a 'chord'.     Two or more different notes a 'chord' can often be called two or more 'chords'
Most basic chord to which we give a name is a triad, a three-note chord. Two notes, or even one note, can suggest or imply a Triad, but they cannot fully define it. Two or more different notes a 'chord' can often be called two or more 'chords'