Private income is either:

• any type of income
  Income
  Income is the consumption and savings opportunity gained by an entity within a specified time frame, which is generally expressed in monetary terms. However, for households and individuals, "income is the sum of all the wages, salaries, profits, interests payments, rents and other forms of earnings...
  
   received by a private individual or household, often derived from occupational activities, or
• income of an individual that is not in the form of a salary
  Salary
  A salary is a form of periodic payment from an employer to an employee, which may be specified in an employment contract. It is contrasted with piece wages, where each job, hour or other unit is paid separately, rather than on a periodic basis....
  
   (e.g. income from investment
  Investment
  Investment has different meanings in finance and economics. Finance investment is putting money into something with the expectation of gain, that upon thorough analysis, has a high degree of security for the principal amount, as well as security of return, within an expected period of time...
  
  s).

Many people with a private income of the second type are comfortably or well off and may not need a job for additional income. People in such a position can have other pursuits, e.g., as a philanthropist

Philanthropist

A philanthropist is someone who engages in philanthropy; that is, someone who donates his or her time, money, and/or reputation to charitable causes...

, or in the past as a gentleman scientist

Gentleman scientist

A gentleman scientist is a financially independent scientist who pursues scientific study as a hobby. The term arose in post-Renaissance Europe but became less common in the 20th century as government and private funding increased.-History:...

. Today among America's 146,000 highest income households, those making more than $1.6 million per year, 31% of all income was derived from investments.