The IRS defines income quite specifically.
On the topic What is Taxable and Nontaxable Income, they note:
You can receive income in the form of money, property, or services. This section discusses many kinds of income that are taxable or nontaxable. It includes discussions on employee wages and fringe benefits, and income from bartering, partnerships, S corporations, and royalties.
Bartering, or giving someone wages (or similar) in something other than currency (or some other specifically defined things, like fringe benefits), is taxed at fair market value:
Bartering
Bartering is an exchange of property or services. You must include in your income, at the time received, the fair market value of property or services you receive in bartering. For additional information, Refer to Tax Topic 420 - Bartering Income and Barter Exchanges.
Bartering is more specifically covered in Topic 420 - Bartering Income:
You must include in gross income in the year of receipt the fair market value of goods or services received from bartering. Generally, you report this income on Form 1040, Schedule C (PDF), Profit or Loss from Business (Sole Proprietorship), or Form 1040, Schedule C-EZ (PDF), Net Profit from Business (Sole Proprietorship). If you failed to report this income, correct your return by filing a Form 1040X (PDF), Amended U.S. Individual Income Tax Return. Refer to Topic 308 for information on filing an amended return.