Net income is defined in the tax laws to mean net income as calculated for tax purposes. 
DEFINITION of 'Net Income-NI'. 1. A company's total earnings (or profit). Net income is calculated by taking revenues and adjusting for the cost of doing business, depreciation, interest, taxes and other expenses. This number is found on a company's income statement and is an important measure of how profitable the company is over a period of time. Yes. A self-employed individual is required to report all income and deduct all expenses. Revenue Ruling 56-407, 1956-2 C.B. 564, deals with the issue of taxpayers not taking all allowable deductions in computing net earnings from self-employment for self-employment tax purposes. Rev. Rul. The Internal Revenue Code requires you to pay taxes on most types of income. Even some non-monetary compensation meets the definition of income for tax purposes. Certain types of income are exempt from income tax or qualify to have the tax deferred to a later date. Income is different to net income. Net income is defined in the tax laws to mean net income as calculated for tax purposes. Income is not defined. To apply section 97 it is necessary to know what the income of the trust is and what a beneficiary’s share of that income is.
http://www.investopedia.com/terms/n/netincome.asp https://www.eitc.irs.gov/Tax-Preparer-Toolkit/faqs/income http://www.ehow.com/info_7802318_definition-income-tax-purposes.html http://www.bartier.com.au/publications/publicationDetail.aspx?PublicationID=240
define net income for tax purposes