Despite the varied and interesting responses that followed I think a much simpler answer could be offered:
The DCC 'standard' is accepted as being that set by the NMRA.
The NMRA is based in the USA.
The NMRA adopted the term 'consist' to describe two or more locomotive receiving identical speed, and direction, commands from a single throttle.
The concept is quite simple to understand and the word is nicely concise.
The concept is not restricted to two locomotives, as is implied by 'double heading'.
As far as I'm aware no prototypes receive traction and/or control via a DCC system and so it can easily be argued that esoteric discussions about modelling fidelity don't apply :-)
Bottom line, "You need to understand DCC terminology, it helps if you understand North American practise.".