Basis (or cost basis), as used in United States tax law, is the original cost of property, adjusted for factors such as depreciation. When property is sold, the taxpayer pays/(saves) taxes on a capital gain/(loss) that equals the amount realized on the sale minus the sold property's basis.

Cost basis is needed because tax is due based on the gain in value of an asset. For example, if a person buys a rock for $20, and sells the same rock for $20, there is no tax, since there is no profit. If, however, that person buys a rock for $20 and then sells the same rock for $25, then there is a capital gain on the rock of $5, which is thus taxable. The purchase price of $20 is analogous to cost of sales.

Typically, capital gains tax is due only when an asset is sold. However the rules for this are very complicated. If tax is paid because the value has increased, the new value will be the cost basis for any future tax.

Internal Revenue Service (IRS) Publication 551 contains the IRS's definition of basis: "Basis is the amount of your investment in property for tax purposes. Use the basis of property to figure depreciation, amortization, depletion, and casualty losses. Also use it to figure gain or loss on the sale or other disposition of property."