What is Fair Market Value?

Legal Definition
Fair market value (FMV) is an estimate of the market value of a property, based on what a knowledgeable, willing, and unpressured buyer would probably pay to a knowledgeable, willing, and unpressured seller in the market. An estimate of fair market value may be founded either on precedent or extrapolation. Fair market value differs from the intrinsic value that an individual may place on the same asset based on their own preferences and circumstances.

Since market transactions are often not observable for assets such as privately held businesses and most personal and real property, FMV must be estimated. An estimate of Fair Market Value is usually subjective due to the circumstances of place, time, the existence of comparable precedents, and the evaluation principles of each involved person. Opinions on value are always based upon subjective interpretation of available information at the time of assessment. This is in contrast to an imposed value, in which a legal authority (law, tax regulation, court, etc.) sets an absolute value upon a product or a service.

An eminent domain taking, in lieu of a property sale, would not be considered a fair market transaction since one of the parties (in this case, the seller) was under undue pressure to enter into the transaction. Other examples of sales that would not meet the test of fair market value include a liquidation sale, deed in lieu of foreclosure, distressed sale, and similar types of transactions.
-- Wikipedia
Legal Definition
The value of property as determined by the market place (or objective purchasers) rather then as determined by a subjective individual. This is what an informed and unpressured buyer would pay to an informed unpressured seller in an arms length transaction (the price is based solely on the value of the property, as opposed to if you were selling the property to a family member and giving them a special deal).
Legal Definition
The price a buyer will pay. All parties are willing and aware of the property and its value.