The semantics would have it that three notes are needed to be a 'chord'. Two notes together are, technically, an 'interval'. The loose expression 'power chord' has crept into modern parlance, and is grudgingly accepted with a nod to rock folks, but definitions are definitions, so, pedantically, one would say that three notes are needed, at a minimum, to have technical 'chord' status. The debate will doubtless rage on, however, and the tectonic plates continue their voyage.