I think the term "passive income" might be a marketing gimmick because it makes people think of "free money for no work". I suspect many online charlatans have adopted the term solely for this get-rich-quick emotional response that it generates in the unsuspecting. Like "ooooh he's going to teach me how to make money without doing anything! I'll pay for that!"
The only way I can think of to generate income without doing anything at all is to earn interest on interest. To earn interest in the first place you have to work to make money to invest, but once that investment starts earning interest, the interest also goes to work for you passively earning more interest.
Right. I choose to put a portion of my earnings in an index fund, while other choose to put theirs in a savings account, or to spend it entirely. It takes me no more work to have money automatically deposited in my retirement accounts than it takes them to have it put in their checking accounts. Yet last month my net worth increased several hundred dollars simply because of where I chose to put my money. It's hard to get more passive than that.
I think you can divide rental income into earned and passive aspects. If you invest in a rental, and make a profit from it after all related expenses are covered, it's passive income. You could have a portion of the rent go automatically to a property manager's salary, and have to do 0 work to receive the surplus. Or, you could hire yourself as property manager, pay yourself a nice wage to do the work, and be generating earned income and passive income from the same source.
The only other source of income, besides an inheritance, that could be considered passive would be alimony (some would say child support, but raising a child is not exactly a passive pursuit, and you have to be participating in the child raising to receive child support), or structured settlements.