There is no standard definition of "net income." It means different things to different people, and in different contexts. You have to decide yourself what you mean by net income, for whatever purpose you are using it for. If someone is asking you what your net income is, you have to ask them exactly how they define it.
The TurboTax article that mesquitebean referenced says "Net income typically means the amount of income left over after you pay your income tax or get a tax refund." Right away, the word "typically" tells you that it doesn't always mean the same thing. There are many different types of income. Some is taxable, some is not taxable, and some is partly taxable. The income on your Form 1040 can even include money that you didn't actually receive. You have to decide what kinds of income you want to include in your "net income." And despite the article's emphasis on after-tax income, net income often means income before tax. If you are looking for after-tax income, you also have to subtract state tax. And net income is not the same as take-home pay.
The Accounting Insights article that mesquitebean referenced dances around total income, AGI, taxable income, and various schedules, but never tells you what net income means or how to calculate it. The article basically considers net income to be before tax, though it is not entirely consistent about that.
So the answer to your question is that there is no line on Form 1040, or anywhere else on your tax return, that is "considered" net income. You have to decide what you mean by net income, and calculate it yourself.
