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Exchange (of contracts)
At exchange, both buyer and seller officially agree that the sale will take place. Signed copies of the sale contract change hands, and a deposit is often paid. A completion date is agreed in advance of the exchange.
Once exchange has taken place neither side can back out (without serious legal and financial consequences) so this is the point that all uncertainty is over.
For this reason also it is critical that your lawyer complete all the important work before you exchange.