Fair market value (FMV) is the price that property would sell for on the open market. It is the price that would be agreed on between a willing buyer and a willing seller, with neither being required to act, and both having reasonable knowledge of the relevant facts.
Source: IRS Publication 561
This definition is the standard the IRS uses to determine if an item sold or donated by you is valued properly for income tax purposes. It's also a definition that's open to interpretation. If the IRS decides your FMV opinion is incorrect, you're not only subject to more taxes, but also penalties to boot.
Here are some tips to help defend your FMV in case of an audit.