The IRS defines passive income as a rent recieved on a regular basis with little effort required to maintain it.
Here are some examples of ways you can earn passive income:
Some examples of passive income are:
Earnings from a business that does not require direct involvement from the owner or merchant;
Rental from property;
Royalties from publishing a book or from licensing a patent or other form of intellectual property;
Earnings from internet advertisements on websites
Residual income, repeated regular income earned by a sales person, generated from the payment of a product or service, that must be renewed on a regular basis in order to continue receiving its benefits
Pensions