There are those who say it isn't a chord until you have 3 notes. I'm going to skip the complex music theory behind the arguments (I haven't studied them enough to understand much less form an opinion).

Of course in most music contexts there is more than one instrument playing and so the others can fill in the missing note making a full chord. Only a "one-man-band" really feels the loss.

Power chords only have two notes (although more than two octaves, I don't know if you count that)