A synchronous modem is a type of modem that transmits data in a continuous, timed stream, using a shared clock signal to synchronise the sending and receiving devices. Unlike asynchronous modems, which send data in individual characters with start and stop bits, synchronous modems group data into blocks or frames and transmit them more efficiently, without extra timing bits.

This makes synchronous modems faster and more reliable for large or continuous data transfers, such as those used in mainframe communication, leased lines, or enterprise networks. However, they require more complex setup and precise timing coordination, which is why they were mainly used in business, military, and government environments rather than in home computing.