A document published by the Internal Revenue Service (IRS) detailing what types of income taxpayers should consider taxable or nontaxable when filing a return. IRS Publication 525 outlines how employees are to treat income from retirement plans, stock options and fringe benefits; how certain employee types, such as military personnel and clergy, report income; how to report income from business partnerships or investment real estate; how to treat disability, sickness and other benefits.

A taxpayer's income can come from a number of sources other than regular employment, and can include exchanges of property or even bartering. Unless a type of income is specifically exempted from taxation by law, it will be considered taxable income.