I'd define it as the first "fully integrated desktop computer", by "input-operation-output" being combined into one portable machine, needing just a power cord to be fully operable.
Funny enough, by that definition, the majority of the "modern" computers do not meet this criteria.
The problem (as you already mentioned), is that the Original IBM PC 8088 4.77MHz, from 1981. Strictly speaking, could have a large CPU/computer box, perhaps on the floor, the monitor and keyboard on the desk and perhaps other stuff, such as printers/external-disks etc, either on the desk or to the side (not on the desk) somewhere.
But clearly, the original IBM PC, has to be the 'gold standard' definition of a desktop computer. Although even that could be disputed.
But in real-terms, it is generally accepted (I think), that the original IBM PC, is the thing which started the big computer revolution. Where Mainframe/mini computers, moved from being in air-conditioned computer centres, to almost everyone having computers, both at home and on their office desk.
But, other inventions, such as transistors, integrated circuits, microprocessors, PCBs, Hard Disks, Networks, The internet, Graphics/video cards, monitors etc. All played their part as well.
I think really, you can just argue the point, that almost anything, started certain technological revolutions off.
There does seem to be a concept, that programmable calculators, are not really full/proper computers, and just a programmable calculating computer, without some of the functionality of a proper, full general purpose computer.