For households and individuals, gross income is the sum of all wages, salaries, profits, interest payments, rents, and other forms of earnings, before any deductions or taxes. It is opposed to net income, defined as the gross income minus taxes and other deductions (e.g., mandatory pension contributions).

For a business, gross income (also gross profit, sales profit, or credit sales) is the difference between revenue and the cost of making a product or providing a service, before deducting overheads, payroll, taxation, and interest payments. This is different from operating profit (earnings before interest and taxes). Gross margin is often used interchangeably with gross profit, but the terms are different. When speaking about a monetary amount, it is technically correct to use the term "gross profit", but when referring to a percentage or ratio, it is correct to use "gross margin".

The various deductions (and their corresponding metrics) leading from net sales to net income are as follows:

Under United States income tax law, gross income serves as the starting point for determining Federal and state income tax of individuals, corporations, estates and trusts, whether resident or non-resident.

Under the U.S. Internal Revenue Code, "Except as otherwise provided" by law, gross income means "all income from whatever source derived," and is not limited to cash received. Federal tax regulations interpret this general rule. The amount of income recognized is generally the value received or the value which the taxpayer has a right to receive. Certain types of income are specifically excluded from gross income for tax purposes.

The time at which gross income becomes taxable is determined under Federal tax rules, which differ in some cases from financial accounting rules.

Individuals, corporations, members of partnerships, estates, trusts, and their beneficiaries ("taxpayers") are subject to income tax in the United States. The amount on which tax is computed, taxable income, equals gross income less allowable tax deductions.

The Internal Revenue Code gives specific examples. The examples are not all inclusive. The term "income" is not defined in the statute or regulations. An early Supreme Court case stated, "Income may be defined as the gain derived from capital, from labor, or from both combined, provided it is understood to include profit gained through a sale or conversion of capital assets." The court also held that the amount of gross income on disposition of property is the proceeds less the basis (usually, the acquisition cost) of the property.