A kickback is when some gets paid additionally "under the table" for things that are outside of the scope of the agreement. It's the "under the table" part that makes it illegal. If everyone knew about the agreement along with the money exchanging hands then it wouldn't be illegal. The actual exchange of money for information is typical with licensing gigs or manufacture specs (such as ARM, etc). It's the fact that he was probably providing extra information to 3rd party developers which apple wanted secret, or favoring a specific manufacture for a task because he would receive additional income from that manufacture to either choose to go with them, or look the other way when they were breaking rules ... either way, introducing additional compensation for any of those activities is what makes it sooooo much worse than him just doing those activities... which alone would have gotten him fired at Apple.
Oxford's definition. I have a Mac too. LOL