Definition of Revenue
Revenue is the amount earned from a company's main operating activities, such as a retailer selling merchandise or a law firm providing legal services.
Definition of Gain
In accounting, a gain is the result of a peripheral activity, such as a retailer selling one of its old delivery trucks. A gain occurs when the cash amount (or its equivalent) received is greater than the asset's carrying amount, which is also referred to as the asset's book value. For example, if the company receives $3,000 for the old delivery truck, and the truck's carry amount (book value) at the time of the sale was $600, the company will have a gain of $2,400.
Definition of Income
Non-accountants might use the term income instead of the word revenue. Generally, accountants use the term income to mean "net of revenues and expenses." For example, a retailer's income from operations is its net sales minus the cost of goods sold minus its selling, general and administrative expenses.