A simultaneous exchange is a 1031 exchange in which the relinquished and replacement properties close on the same day.
In a simultaneous exchange the sale and the purchase happen on the same day, so there is no waiting period between them and the deadlines are not an issue. It is much less common today because it is hard to coordinate two closings at once, and most investors use a delayed exchange with a qualified intermediary. Even when timing is tight, a simultaneous exchange still involves specific documentation, so plan with your intermediary and escrow officer.