Taxable value is best described as:

The definition of taxable value is indeed best captured by the idea that it is the value placed on a property by a governmental unit for tax calculation. This value serves as the basis for determining how much property tax an owner will owe.

Governmental units, such as local municipalities or counties, assess properties to establish this taxable value. They often factor in various elements, including the property's characteristics, location, and market conditions, to determine its worth for taxation purposes. This assessment is crucial for funding public services, infrastructure, and schools, making the taxable value a fundamental concept in property taxation.

In contrast, the other options describe different concepts. The assessed value of property for income tax purposes would suggest a different context that does not directly relate to property taxes. Market value pertains to the price a property might sell for in the current market, which may not reflect its taxable value determined by the government. Lastly, the estimated selling price of a property refers to an expectation or opinion of what it might sell for, which is also distinct from the officially assessed taxable value used for taxation.