There is a specific tax definition of passive income, known as “passive activity” to the Internal Revenue Service. Passive income is any income you make without actively working or are materially involved. The IRS defines it as any rental activity or any business in which the taxpayer does not “materially participate.” Nonpassive activities, or active activities, are businesses in which the taxpayer works on a regular, continuous, and substantial basis.
As doctors, we most likely fall into the first category. We may be rich in the sense that our salaries are in the higher economic range, however, because of our expenses (houses, cars, student loan debt, private school tuition, practice overhead, etc.) and sometimes poor decisions, we have a tough time accumulating any real wealth. We’re also handicapped by the fact that we start along this financial journey relatively later in life.

In equity valuation, residual income represents an economic earnings stream and valuation method for estimating the intrinsic value of a company's common stock. The residual income valuation model values a company as the sum of book value and the present value of expected future residual income. Residual income attempts to measure economic profit, which is the profit remaining after the deduction of opportunity costs for all sources of capital.