Reigeluth (1993) ID definition:

In the Siemens article, he quotes Reigeluth's definition of ID as: " A discipline that is concerned with understanding and improving one aspect of education: The process of instruction". And as Siemens goes on to suggest, this is too narrow a field for my liking, despite my appreciation for much of what Reigeluth suggests in other docs.
That said, there can be a major effect on the learning process of the accurate and effective design of the instruction.