Transmitting data one bit at a time (one bit after the other). Contrast with "parallel transmission," which uses multiple lines to transmit several bits simultaneously (8, 16, 32, etc.). External communications networks use serial transmission.

Serial and Parallel
The internal channels within the computer are typically parallel. The channel between the CPU and memory is parallel, and the channels between the CPU and peripheral devices are typically parallel; however, parallel is giving way to serial (see parallel vs. serial). For example, ATA disk drives (IDE drives) used parallel pathways for years, but Serial ATA superseded Parallel ATA. That might seem like a step backward, but serial circuits are easier to design than parallel, and today's chips are fast enough to make serial a more viable channel/bus technology. See serial port and Serial ATA.