Though it can take a while to build up enough cash to put a 20% down payment on an investment property (the typical lender minimum), they can snowball fairly quickly. The key here is to correctly project income and expenses in order to calculate cash flow (the free cash you can put in your pocket after all associated property expenses have been paid). However you have to be sure to include the cost of a property manager in your calculations unless you want to manage the property yourself. Even with a property manager, you may be required to make large repair decisions every now and then – so while this is not a 100% passive activity, you are not directly trading your time for money like traditional employment.
Residual income is the amount of net income generated in excess of the minimum rate of return. Residual income concepts have been used in a number of contexts, including as a measurement of internal corporate performance whereby a company's management team evaluates the return generated relative to the company's minimum required return. Alternatively, in personal finance, residual income is the level of income that an individual has after the deduction of all personal debts and expenses have been paid.