Definition of 'Boot'
Cash or other property added to an exchange or other transaction in order to make the value of the traded goods equal. Cash boot is allowed to be part of a nonmonetary exchange under U.S. Generally Accepted Accounting Principles. However, for the exchange to qualify as nonmonetary, the value of the boot should be 25% or less of the total fair value of the exchange.
Investopedia explains 'Boot'
When you trade in an older car and also pay cash for a new model, the cash you pay in addition to your older car is the boot.
Boot might also come into play in a 1031 exchange. Because it is difficult to find two like-kind properties of identical value to exchange, one party will commonly contribute cash and/or physical property to even out the exchange. The base amount of the exchange remains tax deferred, but the boot is considered a taxable gain. Even with the boot, however, the recipient will pay less in capital gains taxes for the current tax year than if he had sold the appreciated property and then purchased a different property.